There's a general principle that very small doses of toxins or stresses of any kind - vaccines, radiation, oxidants, poisons, alcohol, heat, cold, exercise - are beneficial, because they provoke the body to a protective overreaction. One of the talks at the 2007 DC conference on cognitive aging even suggested that this is responsible for why people who think more have fewer memory problems as they age.

(This suggests that our bodies are lazy - they could maintain themselves better than they do on every dimension. Or it might be that, if we measured all the responses simultaneously, we'd find that mounting a protective response to radiation made us more vulnerable to infection, alcohol, and all the rest.)