Astronauts get sick more easily in space — 15 of the 29 Apollo astronauts, for example, contracted bacterial or viral infections either on their mission or shortly after returning [source: Young]. That's because space has a bad effect on their immune systems. In particular, microgravity seems to interfere with the activation of T-cells, a type of white blood cell that helps to protect the human body against infections.
On Earth, when a person is exposed to a virus or harmful bacteria, a signaling system known as the PKA pathway turns on 99 different genes, which tell the T-cells to attack the threat. But in a 2005 study, researchers at the University of California-San Francisco found that simulated microgravity can shut off 91 of those genes. The effect was so severe that the researchers compared it to a person having an HIV infection. The problem could worsen on long interplanetary spaceflights, because microgravity and radiation would combine to exacerbate the damage [source: Young].