Thanks to everyone. I think people stay home when they're sick primarily because it's one of the few occasions for taking time off.

I think it's also embarrassing to smell of sick sweat and vomit and to have snot and phlegm coming out of every hole and to have to sprint for the toilet with nausea or diarrhea every 15 minutes. It's weird that so few respondents thought this was a motivator for people to stay home.

I also think that people stay home to rest -- either they are exhausted, or they know that rest will help them heal.

But I rarely talk to people who think of home rest as a way to stop the spread of disease and to take care of others. I usually have to explain the idea when it comes up. I'm surprised how many respondents thought it was a major motivator.

Anyway: first, third, and fourth. Thanks for replying everyone!