Hormesis is a biological response to a substance at very low doses that is opposite the response of the same substance at high doses. In other words, substances that have been shown to be dangerous at high levels might actually be good for you at very low doses. Studies have shown that irradiating a mouse with a very low dose of gamma radiation before subjecting it to a high dose, actually protects it from developing cancer.
The link above is the wikipedia entry and goes into why this is not a popular theory at all. It's not like hormesis is hogwash, even you have heard of it. It is why very small concentrations of botulinum toxin (yes the stuff that causes botulism) injected into our faces makes us look years younger, instead of causing horrible pain and death. Scientists have also described the hormesis effect with opiates. Very small doses of opiate antagonists (pain killer blockers) actually enhance high doses of pain killers. And very small doses of opiates have been shown to induce pain.
There are a lot of government agencies that were designed specifically to protect us (no, not the FBI) that have presumed that the dose-response curves of many substances are linear. As a scientist, I can attest to the pain of measuring a dose-response curve at concentrations below the linear ranges. Still doesn't mean we shouldn't do it. A recent survey based on a review of more than 56,000 tests in 13 strains of yeast using 2,200 drugs indicates that hormesis may actually be a valid phenomenon and dangerous to ignore. Anti-cancer drugs that normally inhibit cell growth actually enhance it at very low concentrations.
Acknowledging the validity of hormesis comes with a whole mess of consequences, however. Environmental groups have advocated completely eliminating toxic substances when that may not be necessary.
But even harder for the government agency types to swallow may be granting credence to the entire field of homeopathy. Homeopathy is a medical practice entirely predicated on treating illnesses with very small doses of substances that at large doses mimic the disease being treated. Maybe there is something behind that whole "crack-pot" theory.