Traveling in space could be hard on your immune system, according to a new study funded by NASA. Researchers tested 25 astronauts before and after various space shuttle missions of varying lengths, and found that white blood cells increased after the astronauts returned from space. The increased number of white blood cells meant that the astronauts bodies were working overtime to fight off various microbes and diseases. And it appears that these effects probably increase as missions get longer and more difficult.