These diseases are generally listed by how dangerous, infectious, or powerful they are. Roll 1d10 for a minor disease, 1d20 for a common disease, and 1d50 for a totally random disease.
50 Fantasy Diseases
This is just trenchfoot. Take 1 point of damage per day of long marching or whenever you run away from a combat encounter.
One of your hands at random has its muscle stiffen up; it becomes very hard to move individual fingers. Instead, it can only roughly grasp things like a "claw". You get -1 to hit with weapons in this hand. If you're a thief, it also takes twice as long to pick locks no matter which hand is diseased.
 Grinning Disease
Gives you a big dopey smile all the time. You're attacked first by demons and evil creatures.
 Wasting Disease
You are always hungry and feel like you're wasting away. You must eat 3x your normal rations each day or lose -1 to your Strength score.
 Bull Snorting Disease
Your airway is restricted. You breathe heavily and give off loud beast-like chuffs on occasion; there is a 1 in 6 chance this happens whenever you try to sneak past someone or hide somewhere out of sight.
 Carnivore Flu
This is a sickness that is exacerbated by eating anything made of plants. You have to only eat meat if you don't want the symptoms to get worse.
 Blue-Dripping Disease
Roll a die to determine a random hole on your character's body. You leak a weird blue liquid from that hole, which stains clothes, hats, etc. This fluid also strangely has a repelling effect on Others- the blue fluid acts as holy water against them, and splashing some of it on them counts as a Turn Undead roll except against an other- with the patient's LVL or HD acting as the power of the roll.
 Frostnipped Nose
Your nose turns blue and gets incredibly cold. Your snot freezes into little icicles that hang down from your nostrils. You have to keep this nose warm if you want to be able to smell anything.
 Salvation Mania
The character becomes very religious. Whatever religion they normally have become amplified, and they start to proselytize about it to everyone; including other party members. Monsters or NPCs encountered with incompatible religious beliefs or the opposite alignment to the character get -1 to reaction checks. If your character is not religious, then they become antagonistically atheist and start to shit on everyone's faiths- causing the same effect.
 Magpie Breath
Your mouth becomes irresistible to birds to make a nest within. They hide all kinds of stuff in there while you sleep; including coins and shiny arrowheads. You must spend an exploration turn after you wake up to floss the twigs out of your teeth, or else you have a 1 in 6 chance to choke on a random copper piece in the back of your throat the next time you try to bark an order or cast a spell.
Like leprosy, but only you tail. Your tail can't feel pain and you get minor infections, people step on it all the time, etc. Every year, your tail has a 1 in 6 chance to fall of, otherwise it is shortened by 20% of its length. If you don't have a tail you're immune to this.
 Chocolate Eye
One random eye fills with a dark brown fluid, making it very difficult to see out of that eye besides very bright light. Gives you -2 to ranged weapon attacks. Every night, there is a 1 in 10 chance a random brownie or some other spirit tries to steal your eyeball and eat it like a snack. You can stop this by wearing an iron eyepatch over it.
 Dangerous Buttocks
Your ass smells bad and produces ungodly flatulence which you have very little control over. If you're out adventuring, add an extra +1 in 6 chance to encounter wandering monsters.
Your facial features droop and sag. You get -1d4 to your Charisma score each year you have this disease. Wrapping your face up in bandages and support wraps to keep your eyes and nose and mouth from sagging down to your chin reduce this to -1d2 instead.
The end of your tongue becomes spiked like a morning star. Talking becomes a slow, careful ordeal. Whenever you cast a spell, you take one point of damage from the complex incantations. You also deal 1d2 damage to anyone if you try to French-kiss them.
 Magic Diarrhea
You have a 1 in 6 chance to release a spell on accident once per exploration turn, starting from the weakest cantrip or 1st level spell and working up. You just can't hold the magic in. If you encounter a wandering monster you can cast this spell as your action on the first round instead of wasting it uselessly the first few minutes before a fight broke out. If you don't have any magic powers then you're immune.
Your bones become weak and brittle. Whenever you take 6 or more damage from a single hit from a blunt weapon, monster fist, or from falling damage you must save or you break a bone in one of your limbs. If a hand, that hand becomes useless. If a leg, you have to hobble around or go on a crutch.
 Terrible Tremors
You are affected by terrible tremors and seizures. Once per day, you have a seizing fits that lasts for about two exploration turns, in which you are totally useless. You won't know when this happens, but your DM will secretly roll a 1d20. If it rolls under your Wisdom score, your character gets an odd feeling that you're going to have a fit in the next hour.
 Smooshing Disease
To everyone else, it looks like you're shrinking. You're not really getting smaller, your body is just smooshing down into a more compact, wider form. Every day this happens you lose some height but stay the same weight, becoming bulkier. Every week, lose -1 to hit but gain +1 AC until you get to 14 natural AC from being a bulkster. You can temporarily regain your normal height by getting stretched out on a torture rack.
 The Tiger Disease
Starting from a slight tan before turning to a dark brown; cartilaginous strips of flesh along your body, especially the torso and midsection, appear. These become stiff and very painful, making your movements less flexible. Treat your Dexterity modifier as -1 while afflicted. After 1d6 seasons, these strips will get so inflexible that they'll burst or rip, causing very painful jagged wounds on your body. Each day, there is a 1 in 6 chance that the strips will rip (with an extra 1 in 6 for each acrobatic stunt you pull or each successful saving throw you make against a trap) and cause you 1d4+1 damage. The name of this disease has a double meaning; first you have stripes like a tiger, and then you look like a tiger raked you with its claws.
 Tamsundi Twist
Your legs are horribly affected by cramps and twisting. Walking around become an exercise to not cross your legs around or fall on your face- slowing your speed and/or initiative by -10%. Additionally, whenever you sleep, your legs want to cross up and fold themselves into a horrible pretzel leaving you helpless. You need to sleep with leg braces or splints on.
 Dragon's Lung
Gives you a dry cough and wheezing whenever you perform physical activity. You heal at half speed. However, you can once per day breathe in and out as deeply as you can and spew forth "fire" breath from the phlegm and inflammed fluids in your lungs; dealing 1 fire damage to whoever you cough it on but dealing 2 damage to you.
 Big Brain Disease
Your brain gets bigger. Your skull swells up causing a large deformity in your skull over several months; any melee attack against you that rolls a 20 splits your skull open and you die. On the plus side your huge brain starts to make you smarter. You gain +1 to your Intelligence modifier.
 Yjeetara Fever
Orange plaques of skin form on the extremities, before moving towards the torso and neck. These growths are painful and itchy, but not directly harmful. Each growth allowed to remain on the body increases the person's resting body temperature, which they adapt to as time goes on. Eventually, their blood will be boiling hot but they will still live. This disease kills by making the person's body so warm they can't digest food or drink water without it vaporizing when it hits their tongue and throat, causing them to die of thirst or hunger.
 Social Awkwardness Disease
Makes you (more) socially awkward. You find fumbling your words, making inappropriate jokes, and trailing off on tangents about things nobody cares about but you. You get -1 to reaction checks and can't use any hero/conviction/bond abilities if your game has those. If you need a more concrete mechanic- then just say that any healing spells placed on you by the Cleric are reduced by -1 hit point because they have some subconscious resistance to being near you.
 The Gray Disease
Your body loses all of its natural color and pigmentation, turning black and white. There's no negative gameplay effects to this; but this disease also spreads to clothes and other items you touch and wear all the time too.
 The Drowning Disease
Having this disease makes your muscles cramp up when you're in water. It has a similar effect with other fluids; 1 in 6 chance that killing an enemy in melee splashes enough blood/ichor on you to cramp up your muscles, making you fail any saving throws that must be down next round. If you ever fall into water, even waist-high water, you will certainly drown without help.
 Larkinson's Disease
Causes random muscle firings and an inability to sit still. Specifically, applies to music. Whenever you hear music, you feel an uncontrollable urge to dance. You get +1 AC whenever dancing around to music, as it makes you very spry, but you must move every combat round and cannot stay in the same place long enough to cast a spell or hold a line, etc.
 Pacifists Disease
You become adverse to any kind of blood or violence. You must make a saving throw at the start of combat; upon failure, you are unable to attack anyone, though you can still cast non-damaging spells, block attacks, run away, etc. Turns your face pale whenever you see something gruesome or get into a fight, giving the nickname of "Whiteface" to anyone with this condition.
Puffs up your lips and eyes, giving you a fish-like look. You have -1 to reaction checks. Every week this disease advances, you feel like it's harder to breath air but feel more and more uncomfortable underwater. After one year, your gills erupt from the sides of your neck and you become an aquatic creature- you can no longer breath air.
 Duke's Disease
You become incredibly egotistical. On the plus side, the body swells with size and strength, granting +1 to your Strength modifier. Your internal organs do not grow to match this, leading to lethargy and eventual death- every week you have this disease you lose -1 to your Constitution modifier.
 Great-Green Cyst Disease
You start to grow glowing green cysts on a random part of your body- you grow one new cyst each month. Each cyst that appears doubles in size every week. If you allow one to grow until it's as big as a fist it produces enough light to see by in the dark, but impedes your movements. Lancing the cysts destroys them but they will keep regrowing until you can find a cure.
 Yin-Yang Essence Leak
Your body begins to release your natural energies as a sweat. If you are female, it will be bright blue Yin energy, that is very cold. If you are a male, it will be bright orange, warm, Yang energy. Every week you have this disease, you lose one level from level drain, until you reach zero level, in which case the fever is miraculously ended.
Other people can drink this essence. Count it as 1% of the character's experience points if you drink or make a potion out of this essence-sweat. The character leaking the essence can drink their own emission to recover that amount of experience. Also if you drink Yang essence as a girl you'll grow a beard and if you drink Yin essence as a guy your cock will fall off, because that's a thing in the chinese light novels where I stole this kind of shit from.
 Troll Flu
You have several flu like symptoms; sweating, shivering, nausea, and a lack of appetite. Also, you turn to stone when exposed to direct sunlight. Don't worry, you'll turn back next time it's dark.
 Lesser Vampirism
You have vampirism, but a really lame, hay-fever sort of vampirism. You get sunburnt if you spend more then an hour outside in the sun a day, and need to drink a few droplets of blood (from someone else) or else you feel weak and listless. If you manage to keep this disease long enough your vampire abilities grow until you'll become a real vampire; roll on the Vampire Bloodlines table to find out what vampire your disease mutates you into.
 Stonelicker Disease
This disease causes weakening bones- especially the ribs. The victim feels very brittle, and any blunt weapons deal double damage against them. If they spend at least two hours per day licking rocks for their vitamins and minerals this double damage is negated for that day.
Your voice becomes very hoarse and grunty. Whenever you try to speak above a whisper, your strained voice becomes the loud baying of a horse.
 Bad Syndrome
Instead of brain there is dark orb. The inside of your head has grown a dark orb which inflicts unwell mental health. Your Wisdom is reduced by -2d6 with a minimum of 1 when the disease reaches its full stage. Additionally, one year after having the disease regardless of its progression, you will experience the sludge, where once per day you must drool or sneeze out a small bucketfull of dark black goo. If left alone and out of sunlight, this goo will metamorphize into a 1 HD Black Snotgoblin. If enough of the sludge is left together in one place, it will likely form into an orc or even a troll.
 Black Finger
This disease begins from the tip of a random finger and slowly spreads to the base. The feeling that finger begins to lessen over time, and then eventually fall off. You lose -1 maximum hit points and Dexterity each time this happens. If you jab someone in the chest very hard with a black finger, they catch the disease too and take -1d4 damage like a melee-range spell.
This disease is characterized by yellow eyes, with the veins gradually yellowing and extending to the whole body. Anyone inflicted with Scorpus begins to see everything as a threat; their fear gland is over productive causing constant stress, paranoia, and anxiety. Characters with Scorpus have -1 to morale checks, or -2 after a year of having it.
 Mad Wizard Disease
Inflicts a form of magical schizophrenia on the victim- forcing them to perceive the world in a disjointed fashion, sometimes seeing or hearing things that aren't there. This spell is characterized by the infected casting "spells" by waving their hands around and saying nonsense words, believing they have magical abilities. People with actual magical abilities are immune.
 Blackgrist Disease
This disease has very few outside effects, making it seem harmless or that no infection is present for the first 1-3 years of having it. During that time, the internal organs slowly shrivel down into black chalky parodies of what they once were. Once the symptoms begin, they are deadly and it is far too late to stop it. Uniquely, the corpses left behind from a Blackgrist infection can have their Blackgrist material broken down and boiled to create a powerful potion base- with the fumes from this reaction acting as a powerful euphoric drug. Every use of the drug or a potion made from Blackgrist confers a 1% chance to catch Blackgrist from them.
 Manead Fever
Prognosis; a high fever with bright red extremities, appearing similar to the tips of fingers and toes dipped in blood. Other symptoms? Incontinence.
 Procrastination Pox
Large purple welts appear on the skin of the individual who has it. Strangely, they don't seem to want to find a cure. Lancing the boils is painful and ineffective; most effective treatment seems to be making the patient actually get off their ass and do something about it.
Your hands get bigger. The flesh of your palm and your fingers below the farthest joint swell up, getting bigger and less useful. After one week, you can't hold a lockpick. After two weeks, no daggers or wands. After one month, you can't hold anything.
Your spine and back muscles contort and twist strangely. Every day you have this disease you lose -10% to your overall carrying capacity, with a minimum of 10% of your normal max. From then on, you lose -1 Strength or Constitution until it finally kills you.
 Ghost Disease
Less of a disease and more of a very clinical form of spirit possession. When your humors are misaligned; this can happen sometimes. The ghost of a dead person has made its residence inside your body; letting them pilot it when you are asleep or unconscious, cause minor "hauntings" like hearing tiny footsteps from your skull, and so on.
Characterized by lowered impulse control, intense wanderlust, violent and sexual tendencies along with addictive behaviors, and growth of body hair. Contrary to the name, most pirates do not actually have this disease and are actually just dicks.
This person's magical energy begins to collect in one of their elbows. Wizards and Magic-Users suffer from this effect faster then normal people; speed up the speed of this condition by the MUs level multiplied by whatever the "normal" amount would be; so a 5th level MU would advance in this disease five times as fast as a muggle.
Whenever you bang your elbow, you accidentally release magical energy. This creates a Wild magic effect, or fires out a blast dealing 1d4 damage to a random person- rolling first for those who are roughly in front of you when you bang your elbow. This magical energy gets more extreme for every month you have the disease;
- One month. As described above.
- Two months. Magic blast deals 1d8 damage + wild magic effects are more extreme.
- Six months. Magic blast deals 1d10 damage + elbow can now fire off your lowest prepared spell without your consent. This spell will still target enemies and try to be useful, but its as useful if you catch it first.
Also; while you have Spellbow the affected elbow grows a bright blue patch on the back to signify it as something special and important to the primitives who tend to catch it.