First time I've seen this outside of a Marvel comic book reference:

Cosmic rays can mutate

Charged particles zipping through space, known as cosmic rays, can mutate the otherwise manageable microbes, spurring the bugs to reproduce quicker and become more virulent, recent studies show.

At the same time, exposure to cosmic rays and the stresses of long-term weightlessness can dampen the human immune system, encouraging diseases to take hold.

Mutant Diseases May Cripple Missions to Mars, Beyond