Cholera is an intestinal infection caused by drinking contaminated water or eating raw meat. There is a very small chance of getting it when eating meat-type foods, even if they're cooked. It causes diarrhea and vomiting which can lead to a severe dehydration and death.
Cholera has three stages. The first and second stages are not so severe, and if proper precautions are taken, such as taking antibiotics or drinking tannin tea, then the disease will most likely not progress further. However, if cholera advances to stage three it can have deadly implications for the player. Every turn will cause the hydration bar to go down one level. If the player has no acess to lots of water to drink at this point, he will most likely die. During stage 3, each turn has a small chance of instantaneously killing the player, regardless of thirst, hunger, or immune system levels.
The Immune System will fight off the infection as long as it stays on a high condition. The higher the Immune System status is, the faster cholera will be cured. The player should also manage the vomiting condition by staying hydrated. It's recommended that the player drinks bottled or sterilized water, since there is always a possibility that collected water could be contaminated, which will make cholera even worse.