The Monster Economy of AD&D

AD&D's most endangered creature.

We all know that larger towns and cities will usually have a market for dragons. That is, it is generally understood that adventurers will occasionally drag a cowardly battered and chained dragon into town, and when they do, there will be wealthy people there ready to bid for the privilege of making the dragon their slave. If you're using a table to determine what's going on in the town or city when your player characters arrive there, you probably already have "dragon auction" on the list, right?

Well get ready, because there is a lot more to AD&D's monster economy than just simple dragon slavery. I thought it would be fun to go through the Monster Manual and find all of the critters that can be enslaved, skinned, robbed of their eggs, or butchered for parts. That's fun, right?

Our first example is not particularly exciting, and not a "monster", per se, but nonetheless offers a bit of opportunity to the enterprising adventurer/trapper. The badger (or giant badger) can be skinned, and its pelt sold for 10-30 gold pieces:

Badgers can be found in dungeons in groups of up to 8. The payout for a successful badger encounter could be anywhere from 10 to 240 gold pieces. It's not much, but every bit counts. 

My next example might seem like more of the same, but there's more than meets the eye when it comes to the giant beaver:

Giant beavers are considerably more intelligent than badgers. They engage in trade, and will agree to construct a dam if offered valuables. Cute! It's like a Disney movie.

Of course, they can be killed and skinned for 500 to 2,000 gold pieces per hide, and their children (if under 8 hit points) can be subdued and sold as slaves for 100 to 200 gold pieces each. There will be 10-40 adults, which will yield 5,000 to 80,000 gold for their pelts alone! There will be an equal number of kits, with about 4 to 18 (44%) small enough to turn into slaves, which is another 400 to 3,600 gold! The kits that are too old to make good slaves are either killed alongside their parents or orphaned and left to starve.

Fire beetle glands can be harvested to shed light in a 10' radius for 1 to 6 days after being removed:

These are "highly prized by miners and adventurers", although we're not told for how much they can be sold. Still, we can imagine that they are considerably more valuable than a simple torch and that they are probably sold in settlements near dungeons (where they are found in the 1st level in groups of 1 to 4).

Blink dog lairs have a 50% chance of containing 3-12 pups which can be captured and sold for 1,000 to 2,000 gold pieces each:

That's 3,000 to 24,000 gold if you get lucky, and all you have to do is slaughter 4-16 Lawful Good intelligent dogs and stuff their puppies into a sack (and, I guess, somehow prevent them from "blinking" away).

Dwarven craftsmen can fashion a bulette's armor plates into +1 to +3 shields:

It's unclear how the exact bonus is to be determined, how much it would cost to have the shield made for you, or how much the dwarves might pay for the materials. I'll return to this a bit later.

The entry on giant eagles is a bit confusing, but I think it means that there is a 50% chance of each nest containing young or eggs (if the initial roll is positive, a subsequent roll is made, with a 50/50 chance of young or eggs being present):

While the entry uses the vague language "they can be tamed" (presumably referring to both young and eggs) only eggs sell for 500 to 800 gold pieces each, leading me to conclude that actually only those newly hatched from eggs can be tamed, and the young already hatched in the nest are worthless. So steal the eggs, but like those giant beaver kits too large to be enslaved, either kill the young in the nest or leave them to die once the adults are slain.

Since giant eagles appear in groups of 1-20, there is one nest per 2 eagles, and 1-4 young/eggs per nest (0 to 40 young/eggs in a lair), knocking over a giant eagle lair can net you anywhere from 0 to 32,000 gold pieces!

Elephant tusks can be sold for 100 to 600 gold pieces:

With each elephant having two tusks, and 1 to 20 or 1 to 12 elephants being encountered in a group, slaying a herd of elephants may yield 200 to 24,000 gold pieces! The tusks are heavy, though - the cheapest weighs 25 pounds, and the most valuable weighs 150 pounds.

Unlike giant eagles, griffon fledglings and eggs are valuable commodities. In fact, the fledglings are more valuable than the eggs:

This suggests, to me, that griffon eggs are difficult to hatch, so it's better to allow the griffon adults to do the work of hatching the eggs and then steal the young fledglings from them. With 2 to 12 griffons per group and 1 or 2 eggs or young per pair of griffons (anywhere from 0 to 12 eggs or young), raiding a griffon lair can yield up to 60,000 gold pieces!

Like griffons, hippogriff eggs and fledglings can be sold as well, and the fledglings are once again more valuable:

Hippogriffs appear in groups of 2 to 16, so a lair might contain as many as 16 eggs or fledglings - up to 48,000 gold pieces!

Fire lizard eggs sell f or 5,000 gold each:

If you happen to find a lair containing 4 eggs, that's 20,000 gold pieces.

Mammoth tusks are 50% more valuable than elephant tusks (150 to 900 gold pieces each - a group of 12 such creatures could yield up to 21,600 gold pieces) but also weigh 50% more (37.5 pounds to 225 pounds), while mastodon tusks are valued the same as elephants':

"Their" here refers to mastodons.

Like giant beavers, giant otters can be skinned for their pelts:

A group of 5 giant otters can yield up to 20,000 gold pieces in value.

Returning again to the matter of eggs and hatchlings, those of the giant owl can be sold as well, and the hatchlings are again more valuable:

A nest can contain up to 3 hatchlings, which will yield 6,000 gold pieces.

Owlbear eggs and young can also be sold:

Again, the young owlbears are more valuable than the eggs. A lair with 6 young yields 30,000 gold pieces. As with the giant beavers, some young will be too large to tame - again these must be either orphaned or killed along with their parents.

Pegasus eggs and young can be sold as well, and again the young are more valuable:

A full group of 10 pegasi might have 10 young, which can be captured and sold for a total of 50,000 gold.

Giant weasel pelts sell for 1,000 to 6,000 gold pieces:

A full group of 8 weasels yields 48,000 gold.

Whales produce ambergris, which can be sold for up to 20,000 gold. Their carcasses can also be sold for up to 3,600 gold:

Groups of up to 8 can appear at once, so this could be 160,000 or 28,800 gold, if somehow you manage to encounter 8 sick whales or can tow 8 whale carcasses into port. Good luck with that!

Winter wolf pelts can be sold for 5,000 gold:

A group of 8 winter wolves would yield 40,000 gold.

These are all of the examples made straightforward by the Monster Manual. Things like wild dogs, horses, and ponies can be captured and trained, and prices for these creatures are provided in the Player's Handbook:

But what of the more "exotic" animals? Elephants "can be trained to carry equipment and/or men", and wolf cubs "can be trained as war dogs or hunting beasts", but no attempt is made to put a value on those animals (a trained wolf would be more valuable than a dog, I would think). In addition to wolves, dwarves can tame brown bears. Gnolls can tame hyenas. Gnomes tame badgers and wolverines. Hobgoblins keep carnivorous apes (my personal favorite monster-pet pairing).

And what of other "fantastic" beasts? Elven maidens might ride unicorns. Wood elves tame giant lynxes. Hill giants keep dire wolves and giant lizards. Storm giants keep rocs. Giant goats "have been tamed to serve as steeds". Hell hounds "are favored as watchdogs by monsters and fire giants". Hippocampi, giant sea horses, and sea lions can "be trained to serve as steeds" or "for use in guarding and hunting".

Presumably the domestication of these animals and monsters is solely the purview of the the peoples and monsters explicitly mentioned, but that doesn't necessarily stop the player characters from profiting from those relationships. Can they sell wolf and bear cubs to dwarves? Giant lizard and roc eggs to hill and storm giants? Hell hound pups to fire giants? How much would those creatures be willing to pay for these commodities? 

All of this killing and skinning and theft and enslavement of young animals and monsters probably sounds pretty Evil - and that's because it basically is. I'm of the opinion that the poaching of animals in the real world (represented here by the killing of badgers for their pelts or elephants for their tusks) is Evil, but if you dig a little deeper into what's going on in AD&D, it's even worse!

Giant beavers are intelligent and engage in trade! Blink dogs are Lawful Good! Giant eagles, while Neutral in alignment, "ignore any good creatures but attack evil creatures" and are of average (i.e., human) intelligence! Giant otters are semi-intelligent and are characterized by their love of play! Giant owls are very intelligent and "sometimes befriend other creatures"! Pegasi are Chaotic Good, "serve only good characters" and "always serve unto death"!

I'm sure the locals who have to deal with bulettes, griffons, owlbears, and winter wolves don't much mind those particular creatures being hunted to extinction, but the majority of these creatures, when not purely animal in intelligence and habits, have a distinct inclination towards Good! 

And I kind of like it. There's something about giving players monetary incentive to be Evil (or at least morally unscrupulous) that's appealing to me. Being Good is hard. It should be. It should be easier (or at least more profitable) to be Evil. Players can write "Good" on their character sheet all they want, but they have to actually do Good for that to mean anything. And for doing Good to actually mean something, it has to be a choice. For doing Good to be a choice, the players must also have the choice to do Evil.

If they spend the whole game doing little quests for the dirt farmers of the local thorp out of the Goodness of their hearts with little wealth to show for it, they may start rethinking what they wrote on their character sheet once they hear that their (Evil) rivals just earned 80,000 gold from skinning giant beavers and selling their children into slavery. Maybe it just reaffirms their commitment to Goodness, and the existence of such Evil in the world instead serves to distinguish them from those sorts, and they vow to put an end to such injustices at any cost. But the (potential for) Evil deeds must exist to create that contrast.

You might have players who choose to play Evil characters, and that's okay too! A campaign of poaching monsters and selling their children into slavery is definitely not for everyone, so you might restrict these options, but I personally enjoy portraying a fantasy world in all its ugliness. If there are giant beavers nearby, the village may be negotiating with them to construct a dam, while simultaneously, outlaw poachers may be conspiring to prey upon them. The pegasi nesting nearby may be valuable allies to Good characters, but if there's a Chaotic Evil wizard in the area, they're likely to desire the creatures' eggs for nefarious purposes, and are willing to pay good money for them. The players get to choose how to engage with those dynamics one way or the other.

It's not all skinning and stealing babies, though. The section in the DMG on the fabrication of potions describes the need for a "special ingredient", which should be of "high or greater" difficulty to obtain:

The list of suggested special ingredients suggests that most of these will come from monsters:

We can still detect a hint of Evil in this list: human thalamus glands, dragon and giant brains (presumably an Evil act to seek these out if the dragon or giant in question is Good), pegasus hearts, and more. For the most part though, the aspiring alchemist will be collecting their ingredients from Evil or adversarial monsters which adventurers will likely battle in the normal course of their escapades: shapechangers, mind flayers, trolls, elementals, undead, beholders, and the like. Unlike the bits I highlighted from the Monster Manual - which is very much concerned with animals and animal-like monsters - the list includes very few animal parts.

These are all suggestions, so the DM can use this list exactly as is or come up with their own special ingredients for potions. Gygax also provides a rule of thumb for determining the cost "to concoct the basic formula - with rare herbs and spices and even more exotic ingredients", which is equivalent to the potion's experience point value:

We see in the magic item tables towards the end of the DMG that a potion's gold piece sale value is greater than its experience point value:

Thus we have two different potion costs - the cost to buy one, and the cost to make one yourself. The increased sale value includes the cost of labor and the seller's profit margin. I assume that the cost to create a potion includes the cost of buying ingredients (including the "special" ingredient harvested from a monster) as well as keeping one's laboratory properly stocked with the more mundane requirements essential to its upkeep. 

So what, then, is the value of the special ingredient? If the player character party consists solely of beefy fighters who have no means of creating potions themselves, but they just got back from a raid on a mind flayer lair and have a barrel of pickled illithid brains (always bring a barrel of pickling liquid), how much can they sell those for?

Unfortunately, the DMG doesn't offer any guidance. My inclination is to look at the percentage markup from the potion's experience point value to its gold piece sale value and instead subtract that percentage from the experience point value. A potion of ESP costs 500 gold to create and sells for 850 - a 70% markup. Thus, a mind flayer brain sells for about 150 gold...maybe? I'm not going to pretend to be a economics guy, but that fairly sounds reasonable to me.

Returning to the example of the bulette, whose armor plates can be fashioned by a dwarven craftsman into a magic shield, a shield +1 has an experience point value of 250 and a gold piece sale value of 2,500. A shield +2 is 500/5,000, and a shield +3 is 800/8,000.

Using the above rule of thumb, you could sell a dwarven craftsman bulette plates for 25, 50, or 80 gold, depending on the quality of the plates. Seems a bit low. Is my rationale off? Someone who is good at economics sound off in the comments.

UPDATE (January 16th, 2025): As 2XSwords pointed out in the comments below (back in October, when this post was first published), the XP value of potions (and thus, the amount of gold in materials needed for concocting) probably assumes that the character has already procured the "special ingredient" through adventure. Thus, my whole rational is flawed. Instead, the gold piece value of the special ingredient should probably be some percentage of the markup from XP value to sale value. Assuming that the difference accounts for the cost of labor, the seller's profit margin, and the value of the special ingredient, we could perhaps rule that the value of the special ingredient is, say, approximately 33% of the markup.

Thus, a mind flayer brain probably sells for about 115 gold pieces, and a bulette plate for 750 gold pieces (assuming that the difference between a +1, +2, or +3 shield is a matter of craftsmanship and not the actual quality of the bulette plate itself). 

Just after the section on potions, in the section on the manufacture of scrolls, we learn that each spell transcribed on a scroll requires "a fresh, virgin quill" which "must be from a creature of strange or magical nature":

Gygax goes on to explain that "special quills cannot normally be purchased":

This seems a bit odd to me. As with my previous example of the party of fighters with a barrel full of mind flayer brains they cannot use, surely there will be adventurers slaying all manner of magical feathered monsters. Even if the party includes a magic-user, surely that magic-user will not have need of every single feather plucked from every single griffon. So, why can't they sell them? And inversely, why couldn't they also buy them? How much would they sell for? There isn't much to go off of.

We also learn that the ink used to scribe spells onto scrolls and into spell books is concocted from the ink of a giant squid or giant octopus, plus "blood, powdered gems, herbal and spice infusions" and "draughts concocted from parts of monsters" (emphasis mine):

The example suggests that a unique formula should be devised for each and every spell (or protection scroll) to be scribed. This one includes basilisk eye, cockatrice feathers, and venom from a medusa's snakes. The ink "is compounded only by the inscriber", suggesting it cannot be purchased (perhaps concocting the ink is part of the magical process, so the inscriber must have a part in it). But how much does it cost to compound it? And in turn, how much can you sell those monster parts for?

The only real indication we have as to how much this ink should cost comes from the magic-user spell write which, confusingly, suggests that you can purchase the ink...maybe:

This is a specific use case, so it may not apply more broadly to ink used in scribing spells generally, but I don't see why not. Keep in mind 200 is only the minimum, and as this is a 1st level spell used to scribe spells of at least 2nd level, we might say the gold piece sale value of ink is 100 gold per level of the spell, which is consistent with the rule of thumb used in subsequent editions of the game. 

This makes sense, as if the ink needed to scribe each spell has its own unique formula, some inks would presumably be more difficult to concoct than others. Wouldn't an ink containing dragon's blood be more expensive than one containing only rat blood? Most likely, higher-level spells require rarer ingredients in their ink, which justifies the increased cost.

Here's the footnote beneath the scrolls table in the DMG:

You get 100 experience points per level of spell you scribe onto a scroll, and you can sell the scroll for three times that amount. I would venture to guess that it then costs 100 gold per spell level to concoct the ink yourself, which means concocting the ink for a 2nd level spell would cost 200 gold...which is how much the write spell says the ink sells for. Hmmm...

Perhaps the rule of thumb for potions (cost to fabricate equals experience point value) is unique to potions, and some other equation is at work in the case of other magic items. In the section on fabrication of other magic items, Gygax describes a wizard spending 5,000 gold pieces to craft a ring of spell storing:

However, the magic item tables indicate that the experience point value of a ring of spell storing is 2,500 gold:

As is often the case in AD&D, Gygax seems to want the DM to arrive at a ruling according to individual taste, rather than providing a broader rule of thumb from which to extrapolate. This is fine, but as someone who values AD&D (and the DMG in particular) for its exacting specificity in many areas, it does frustrate me when Gygax is less clear in others.

This is all without getting into material components of spells! Augury requires dragon bones (it's a 2nd level spell, so I imagine the cleric is buying these somewhere, rather than slaying dragons at 3rd level). Scare requires a bit of bone from an undead skeleton, zombie, ghoul, ghast, or mummy. Clairvoyance requires a pinch of powdered pineal gland from a human or humanoid creature. Rary's mnemonic enhancer requires either black dragon's blood or giant slug digestive juice. How much does that stuff cost?

I got a bit into the weeds there, but the point is this - monsters are valuable. They are a part of the world the player characters inhabit, and they are exploited for resources much like animals in our own world, and then some. Their pelts are valuable. Their young are valuable. Their eyes and hearts and brains and feathers and blood are valuable. 

While the Monster Manual's accounting for the value of every pelt, egg, fledgling, tusk, and what have you may seem laborious to some, in my opinion it doesn't go far enough. The DMG suggests that player characters - and the world as a whole - have a much greater use for monsters than just skinning them and selling their pelts.

I don't want to know just how much I can sell the griffon's babies for - I want to know how much a wizard will pay for its feathers. Can I make something with its eyes or talons? How much can I sell those for? Can I take the ankheg's shell and make it into armor? Can I harvest the acid from its guts? How much will someone pay for a suit of red dragon scale mail? 

Just as this is important information for players, it also helps the DM's worldbuilding at a gameable level. I'd be remiss not to mention that in OD&D, castles may be garrisoned by all manner of monsters, as well as Heroes riding griffons, hippogriffs, and rocs:

Clearly they are coming from somewhere! 

If there are valuable monsters nearby, someone in the nearby settlement or stronghold will be looking to exploit them. Just as there will be Evil trappers preying upon the giant beavers and wizards scheming to obtain pegasus eggs, the lord of the castle might be struggling to tame griffon mounts for their riders, the alchemist will pay good money for basilisk eyes, and someone is probably looking to buy a barrel full of slug slime. And of course, there will be the occasional dragon auction. 

The typical market found in an AD&D town or city is a lot more fantastic than you might imagine, and all of those fantastic elements invite engagement by the players. Will they join in on the poaching or plot to put an end to it? Will they make their fortune as slime merchants? Will they infiltrate the Evil High Priest's temple stronghold and attempt to free the gold dragon he just purchased at the auction? The possibilities are much greater when monsters are worth more than just XP.

Advanced Diseases and Diagnoses

Adventuring is dangerous business. Dungeons are filled with monsters, tricks, and traps. The wilderness is littered with small armies of orcs and bandits, and strongholds ruled by hostile high-level characters who demand jousts or tribute in the form of magic items, or send player characters on quests via geas to obtain such items.

But even adventurers who overcome these challenges have another, unseen threat to contend with: disease! The AD&D DMG has detailed rules for handling the contracting of diseases, although Gygax claims that the "system does not attempt a specific treatment of a subject which is beyond its scope and purpose":

From the AD&D DMG, CHARACTER AGE, AGING, DISEASE & DEATH, page 13

I will clarify what exactly Gygax means by this shortly. For now, let's get into the non-"specific" but nonetheless very thorough handling of disease in AD&D.

First, we are told that the DM should generally check whether each player character has contracted a disease once a month, once a week if conditions are favorable, or each time a player character is exposed to a disease carrier:

We are then provided with the percentage chances of contracting a disease each time the check is made:

With a base 2% chance and no situational modifiers, we can expect a player character to contract a disease, on average, once every 50 months, or a little more than 4 years, which seems a bit low. 

In particularly dangerous circumstances - the character is already diseased or infected with parasites (I'll get to that later), is in a crowded, filthy environment (a city, encampment, ship, or siege) in a hot and moist swamp or jungle, and is of venerable age - the chance increases to as high as 14%. On average, such a character would contract a disease about once every seven weeks (remember that under ideal conditions, the check is made every week instead of every month). 

If there is an outbreak in such circumstances and the poor character is being exposed to disease carriers left and right, the chance increases to 24%, meaning the character will contract a disease roughly every four encounters (assuming, I suppose, that there are somehow multiple diseases at play, which seems unlikely in an epidemic scenario). That would be a terrible situation indeed!

Hopefully, the character is a paladin or monk, as both of these classes are immune to disease - in the monk's case, beginning at 5th-level. Speaking of 5th-level, that's also when clerics and druids get access to the cure disease spell, so it's safe to say that disease is, for most adventuring parties, one of those low-level setbacks which is eventually trivialized by access to more potent forms of magic. It is a "lock" for which the players will eventually acquire a "key", so to speak.

And that's great news, because diseases are really annoying! Here is the table you roll on to determine what area of the body is affected by a disease, the disease's occurrence, and severity:

Here we see what Gygax meant when he said that AD&D "does not attempt a specific treatment of" diseases. I take this to mean that specific in-world diseases are not described in detail, but rather generally, and in terms of actual game mechanics. If you want your AD&D diseases to reflect those found in the real world, apparently OD&D Supplement II: Blackmoor has you covered.

Instead of that, Gygax provides some general guidelines regarding occurrence and severity:

And then descriptions of the game mechanics pertaining to each type of disease:

Diseases are brutal. A diseased character is incapacitated for anywhere from 1 to 4 weeks, may suffer permanent penalties to their ability scores, lose the function of body parts, and even die. 

A character who contracts a disease has a 1% chance of it being a brain/nervous system disease, which has a 37.5% chance of being terminal, which has an 8% chance of killing them in one hour. That's a .03% chance overall, so it's very unlikely to happen, but it could. Imagine being the unlucky player whose character drops dead before they even know that anything's wrong with them. People who complain about save or die traps and spell effects in AD&D clearly have not read about the diseases!

If my math is right, a character who contracts a disease has a 13.125% chance of contracting a terminal case - and that's before accounting for modifiers to the severity die roll. Fortunately, having a high (or even average) Constitution score significantly mitigates this risk:

Disease goes hand in hand with parasitic infestation, which the DM must also check for each month or week (in favorable conditions), or whenever the player character is exposed to a carrier:

Parasitic infestation also gets its own percentage chance of occurring:

In particularly nasty situations - the character is exposed to filth (i.e., is in the city, probably), eats improperly cooked meat, drinks polluted water, and is in a swamp or jungle environment - the chance is as high as 16%, indicating infestation once every 6 months, weeks, or encounters with a carrier.

Here is the table of infestation types and severity:

A character who is infested with parasites has a 15.625% chance of developing a terminal case (there are no modifiers to severity for parasitic infection). Assuming an unmodified disease severity roll, parasitic infestations are slightly more deadly than diseases.

A character who dies from disease or infestation (most likely) continues to suffer the effects after being brought back from the dead:

This all to say nothing of lycanthropy, which in AD&D is a disease...as well as a curse? It is treatable with both cure disease as well as remove curse. Lycanthropy however is a much larger subject and is beyond the scope of this post (I will likely write about it some other time).

Aside from lycanthropes and monthly/weekly checks, what else can spread disease/parasitic infestation? Let's start by looking at the Monster Manual.

Cerebral parasites are, of course, parasites. They siphon energy from psionics:

The demon Demogorgon's tentacles inflict a disease which can cause limbs to rot and fall off, or permanently reduce a creature's maximum hit points unless cure disease is administered within 6 rounds:

Another demon, Juiblex, has the spell-like ability to cause disease (the reverse, evil version of cure disease):

Infestation by ear seekers can be thwarted by cure disease if cast with 4-24 hours of contact. Otherwise, the host is 90% likely to die:

Violet fungi's branches inflict flesh rot similar to Demogorgon's tentacles, but that of violet fungi takes effect in only one melee round, although a saving throw is allowed (the mechanical effects of the rot are not elaborated upon, so I'm not sure how it's meant to function in game terms - perhaps similar to that of Demogorgon):

Gas spores, when exposed to flesh, infest a creature with tiny rhizomes (parasites) which kill within 24 hours and produce more gas spores unless a cure disease is applied:

Green slime is a "strange plant growth" which turns exposed creatures into more green slime within 1-4 rounds. Its vulnerability to cure disease suggests it is itself either a disease or parasite, though it isn't clear which:

A bite from a giant leech has a 50% chance of causing a disease which kills in 2-5 weeks (here a mischievous leprechaun from the adjoining column, unaware of or perhaps immune to the danger, is seen riding the leech):

Yellow mold, when attacked, releases spores which infect the lungs and kill instantly unless a saving throw is made. A cure disease and a resurrection cast within 24 hours can save the afflicted:

The touch of a mummy inflicts a rotting disease which kills in 1 to 6 months, permanently reduces a creature's Charisma score by 2 each month, negates cure wounds spells, and reduces the rate of natural healing to 10%. The disease causes the bodies of those killed to rot away, and such individuals cannot be brought back to life unless cure disease and raise dead are applied within 6 turns:

The neo-otyugh inflicts disease, although the method of transmission and the exact type isn't specified:

Presumably, like the standard otyugh, its bite has a 90% chance of inflicting typhus - yes, typhus specifically, for some reason (perhaps the neo-otyugh transmits neo-typhus):

Typhus is, of course, a real world disease, and it affects multiple areas of the body, including the skin, gastro-intestinal organs, and brain. I'm not sure how it would be classified within AD&D's non-specific disease schema.

The bite of a giant rat has a 5% chance of transmitting a "serious" disease (is this meant to mean "severe" disease?) unless a saving throw is made. Since the specifics of the disease are not detailed, presumably the DM would roll on the disease table in the DMG:

Like the cerebral parasite and ear seeker, rot grubs are treated as parasites. Infestation is countered by cure disease if applied within 1-3 turns:

Like the giant leech, a bite from a giant tick has a 50% chance of inflicting a disease which is fatal in 2-8 days unless a cure disease is applied:

Of the monsters that cause disease or parasitic infestation, ear seekers, giant rats, rot grubs, gas spores, violet fungi, wererats, giant ticks, werewolves, and yellow mold all appear on the random monster tables for dungeon levels I thru IV, meaning that player characters will likely encounter these monsters prior to having access to cure disease. Of those, diseases/infestations inflicted by ear seekers, violet fungi, yellow mold, and rot grubs will take their course well before the player characters have a chance to return from the dungeon and treat the ailment, so low-level characters without access to cure disease should be especially wary of these threats.

In addition, while low-level adventurers would be wise not to venture into the wilderness too early, there one might find green slime (in shallow fresh water), giant leeches (in shallow fresh water and uninhabited marshes), lycanthropes (in both inhabited and uninhabited areas), mummies (in uninhabited rough terrain, hills, and mountains - undead are not encountered in deserts, for some reason, and unfortunately there are no bog mummies here), giant rats (in sub-arctic forest, rough terrain, and marshes, as well as most terrain types in inhabited areas), and giant ticks (in uninhabited scrub and forest). While all but the green slime inflict diseases which allow some time for treatment, this is of course the wilderness - who knows how far away treatment might be?

If the player characters happen to use psionics, they run the additional risk of encountering cerebral parasites, demon princes such as Demogorgon and Juiblex, and yellow mold.

It is telling that disease-carrying and parasitic monsters are more common in the dungeon than in the wilderness (dungeons are, fittingly, disease pits).

I would be remiss not to mention the dreaded beggar, which may also be a disease carrier:

Since beggars can be disease carriers, an encounter with one could be cause for the DM to immediately check to see if the player character has contracted a disease (with +10% chance of contracting the disease, according to the table at the beginning of this post) - just another hidden danger of city encounters! And don't forget that giant rats and all manner of lycanthrope will be encountered in cities and towns as well.

Evil clerics - and druids, apparently - can also cause disease by reversing the cure disease spell (druids are Neutral by necessity, but they still have access to the - evil - reverse of cure disease):

Evil clerics can also reverse the heal spell to instead harm and by doing so inflict disease:

It's not all monsters, wretched beggars, evil clerics, and the ubiquitous peril of living in the AD&D world. Cursed scrolls have a 25% chance of inflicting a disease which kills in 2-8 turns:

The periapt of foul rotting, which may appear as any "gem of small value" inflicts a rotting disease which reduces Dexterity, Constitution, and Charisma by 1 each week. When any score is reduced to 0, the character dies. The disease can only be removed by remove curse and cure disease and heal, limited wish, or wish:

Outside of avoiding hot, moist, crowded conditions, swamps, jungles, cities, encampments, ships, nasty monsters, beggars, scrolls, and gems of small value, never getting old, living in the mountains, being naturally gifted in Constitution, or being a paladin or 5th-level monk, are there any other ways to avoid diseases and parasites?

You can start by cooking your meat and boiling your water. Alternatively, a 1st-level cleric can instantly purify it for you:

Druids can only purify water (not food), for some reason:

Additionally, amber gemstones are reputed to ward of disease:

Unfortunately, Gygax immediately informs us that, actually, that's hogwash, and carrying around such a gemstone "will convey absolutely no benefit of magical nature":

This is too bad, as I quite like the list of REPUTED MAGICAL PROPERTIES OF GEMS. I really like the idea of player characters carrying around a bunch of bejeweled items (rather than selling them) in order to benefit from minor magical properties. I say if they're carrying around some amber, give them a -1% chance of contracting a disease!

The periapt of health (which looks exactly the same as a periapt of foul rotting) confers immunity to all diseases except the foul rotting inflicted by the aforementioned periapt (interestingly though, the periapt of health can be crushed into dust and sprinkled upon an afflicted character to counter the periapt of foul rotting's effects):

Finally, one possible minor benign power of an artifact or relic is to grant immunity to disease:

But you're not a paladin or 5th-level monk. You don't live in the mountains. You're of a sickly disposition, and very old. You live in a city which is for some reason built in a swampy jungle. You run into beggars on your way to the plague rat-infested ship which takes you to the dungeon full of gas spores, green slime, and violet fungi. You break for lunch and eat and drink nothing but raw otyugh meat and polluted dungeon water. You're gonna get sick. Where can you get a cure?

Cure disease is of course the obvious solution. As I mentioned earlier, it's a 3rd-level spell for clerics and druids:

In addition to being immune to disease, paladins can also cure disease (even before they gain access to the cleric spell, which isn't until 13th-level):

A psionic can use cell adjustment to cure disease as well:

If you have a disease of the eyes, cure blindness (another 3rd-level cleric spell) will help, but not if you've already lost the affected eye(s):

If you have no cleric, druid, paladin, or psionic in your party, you will have to find an NPC to help you. Cure disease costs 1,000 gold pieces:

A dungeon room containing unguarded gold contains 250 gold per level, so this is equivalent to four such rooms for a 1st-level character and one such room for a 4th-level character (that is, it is not cheap):

Remember that even if the player characters can afford this service, actually obtaining it from the NPC should still be irritating!

If you have no luck finding a human/demihuman spellcaster to aid you, you might seek out a "tribal spell caster":

It will have to be a bugbear, gnoll, kobold, orc, giant, goblin, hobgoblin, or lizard man, however, as these are the only monsters able to attain a high enough spellcasting level:

However, considering that most of these monsters are Evil, and curing disease is Good, casting these spells often will put these monsters in jeopardy with their gods and alignment:

It is probably fair to say that convincing these monsters to utilize this magic will be even more expensive and irritating than usual!

If you also have no luck finding a tribal spell caster, consider tracking down a werebear:

Or steal a night hag's periapt forged in Hades:

Failing all that, you might track down some medicinal herbs, spices, or vegetables. APPENDIX J has a detailed list of such things and their reputed medicinal benefits, which is much too thorough to get into here. In short, they can be used to treat urinary, venereal, skin, liver, pulmonary, eye, mouth, and respiratory diseases. As with gems, Gygax spoils the fun by saying, no, actually, none of this stuff actually does anything, but we don't always have to listen to the old man.

Finally, there are a few magic items that cure diseases, such as the staff of curing:

Keoghtom's ointment:

A bead of curing (found on a necklace of prayer beads):

The cup and talisman of Al'Akbar:

And as a major benign power of an artifact or relic:

All this is to say: give your player characters diseases! Make them face down disgusting little dungeon creatures. Make them ward off hoards of beggars. Introduce a plague to your next urban or seafaring adventure. Have the Evil warlord in the castle lay siege to the city and start catapulting diseased corpses inside. Make the player characters properly cook their meat, or make the cleric prepare purify food and drink. Make them spend tons of gold dealing with annoying NPCs. Make them seek out and parley with monstrous spellcasters and werebears. Make them steal from hags. Make them quest for rare magic items to ward and cure themselves (and don't forget to sprinkle in the occasional cursed scroll or gem). 

Put in them the fear of swamps and jungles, cities, ships, old age, fungi, mold, and rats, and all the unseen dangers lurking in heaps of filth, standing water, rotting wood, and the very air they breathe!