Friends of mine who have taken cruises consistently mention the very careful measures that cruise ships take to prevent illness among passengers. Not infrequently, entire voyages are ruined because of widespread sickness among the passengers.
It makes sense that cruise ships would be at risk for this, since there are lots of people in an enclosed space. But then I wondered, how is this not even more of a problem aboard the ISS? After all, it's a hermetically sealed place, it's difficult to bathe effectively, and based on what I've read about the toilet situation, it would be hard to keep fastidiously clean.
This question addresses communicable diseases. Basic diarrhea seems like a more run-of-the-mill type of illness; I don't know whether it would be possible to detect whether someone was going to have an incident in the next few weeks. And at that point, why wouldn't it just spread all around the station and remain an ongoing problem?