the chronic sinus problems in space are more due to fluids not draining from people's heads than to infection.

A few years ago on the old MSNBC Space BBS, we used to have a regular poster who'd been a backup astronaut during the Apollo program. His name escapes me at the moment. Anyway, we discussed infectious disease in space at one point and he told us about a project where he'd served with a small group of men in Antartica for several months. After a short time, everyone had been exposed to whatever germs the others were carrying. As a result, they didn't get sick any more. I suspect the same would be the case on a long space station stay or on a trip to Mars (moon, whereever). You can't catch an infectious disease without a source.

Of course, if a long space stay weakens someone's immunity, then they may be more likely to get sick after they return home.