Microgravity, cosmic radiation, and human stress can cause some gut bacteria to become stronger and others to become weaker, resulting in conditions similar to traveler’s diarrhea or worse. Microgravity can also prevent bacteria from communicating with one another and with the human cells around them, which could cause some kinds of bacteria to grow out of control. The microbiome provides a first line of defense against infections, but if defender bacteria don’t survive in space, that could make humans more susceptible to diseases like E. Coli. Some studies have shown that the immune cells themselves undergo a change that makes them less effective in staving off disease. With so many people living in a confined space and their immune systems compromised, it’s easy to imagine that these sorts of diseases could run rampant.