Exposure to a large dose of something that causes bodily damage -- such as excessive heat, poison, or nuclear radiation -- is of course harmful to an organism
But, surprisingly, exposure to small doses of such stressors has been shown to extend life span in various species, including fruit flies, protozoans, worms, and rodents
Exposure to small doses of stressors stimulates an organism's natural repair mechanisms to fix any damage caused by the stressors as well as some unrelated damage.