Thanks Lynne, I hadn't heard of this phenomenon.
Here's an excerpt from Wiki:
The biochemical mechanisms by which hormesis works are not well understood. It is conjectured that a low dose challenge with a toxin may jump start certain repair mechanisms in the body, and these mechanisms are efficient enough that they not only neutralize the toxin's effect, but even repair other defects not caused by the toxin. This is similar in principle to viral vector vaccines under development for diseases such as cancer and AIDS.
A deeper explanation is that low doses interact with genetic signaling systems that upregulate gene expression, whereas high doses cause overt toxicity. This fits well with ideas from the evolution of aging. Aging is not a wearing-out or a failure of the body, but rather a purposeful adaptation, the purpose of which probably has to do with population regulation. The body is programmed to self-destruct, but in times of hardship (when many individuals are dying of external causes) the aging program lets up in order to moderate the death rate. The combination of aging and hormesis acts to level out natural population cycles, by keeping the death rate more constant.