Bacteria living zero-gravity environments become more virulent. People living in zero-gravity environments have less-than-fully-functional immune systems. The result is a danger for space travelers that few of us on Earth ever think about — even though a lot of early astronauts, right up through the Apollo program, suffered severe infections in flight, or shortly after landing. Ed Yong's article for Wired UK from 2011 is a reminder that there's a lot of details that need to be worked out before humanity can become a space-faring species. We've got more worry about up there than just radiation.