Now, being in space is much the same, except that the effects don't remain when someone returns to normal gravity--as far as we know at least, considering we haven't exactly stuck someone up there for fifteen years to see what happens. So, your immune system weakens, those little T-cells and their receptors shut off, and your body becomes a happy breeding ground for all the little bacterium and microbes that float around in our air and make homes in our bodies.
This is where a new report from Universe Today comes in. Apparently bacteria--such as salmonella, which was used in this experiment--actually become more dangerous after spending considerable time in space. Astronauts took with them a group of salmonella filled containers with nutrients for the bacteria. What they discovered upon return to the ground was that the bacteria had changed expression in 167 of its genes and become three times more likely to infect. That's surprising when you think about it. Imagine having that floating freely in a space ship with a group of people who are immuno-depressed!
Scientists think it isn't directly related to the zero-gravity, not like it is in us humans, but it is indirectly related to it. They believe it has more to do with the movement of fluids, which on Earth is very low, but in space is probably considerably more active. Now that's some interesting news, eh?
Edit: I changed the phrase "altered 167 of its genes" because it was brought to my attention that that phrase may be confused with mutation, when that was not the meaning I was intending.