Fighting on a bad stomach, being ill in a dungeon.

Had a bad day yesterday, my stomach was killing me. Fortunately it wasn't Moctezuma's revenge, but I did feel terrible. Some food poisoning from something I ate for breakfast.

This got me thinking how often do characters in your campaigns get sick. It is possible to carry iron rations for a week or two, but water? Where do characters get water in a dungeon or wilderness area? How do they purify it? How does this affect your cleric spell list?

Hunting for food and not cooking it properly can pose a whole set of risks to the character's health. From short term infection to long term parasites. Have you taken into consideration these aspects in your campaign?

If you do, how do they relate to game mechanics, hit points, health, spell casting abilities and combat?