This is a tangent on my meta-narcotics post and only really half, or the start of, a mad, little idea. Sickness, illness - having your character contract and deal with a disease in a game is rarely very exciting from a mechanical or roleplay perspective. It could be... much goofier. Having your character take one-off or cumulative stat damage is dull and forgettable, we could make it more engaging and present in everyday play - we could make it META. So;
A disease is made of three different components; a symptom, one or more triggers and an effect. I will show this with the following example:
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Oh no! The adventurer, Gorto Gold-Groveler, has contracted Dungeon Lung! Now Gorto's player (not Gorto the character) must pretend to wheeze before they try to speak to an NPC. Each time Gorto's player does not wheeze or forgets to do so, Gorto will lose 1 HP permanently until his sickness is cured. When cured, Gorto will regain any lost HP. If uncured, Dungeon Lung becomes progressively worse and soon Gorto's player will have to pretend to wheeze whenever they make an ability check. The Dungeon Lung may even progress to a final stage, where the player must wheeze if they are making Gorto do anything other than lay prone. If another player tries to remind Gorto's player to wheeze whenever there is a trigger, that player has opened themselves to risk of contagion and must make a save against becoming infected with Dungeon Lung themselves.
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Does that make sense? While the symptom, trigger(s) and effect can be remixed and swapped out entirely, the core of the idea is - the illness's symptom must be performed the afflicted player whenever a trigger is encountered during play, if not, the afflicted character suffers from the disease's effect. This way, disease becomes more interactive and present for the afflicted player and the rest of the table. With this method, a disease has no other effects than when those caused by a trigger - either the player acting out the illness's symptom or forgetting to do so and suffering an effect. For me, it simply simulates the experiences and feel of actually being unwell - and it is funny and a little gross. Be weird, try it.