Toxicologists have long held that the dose makes the poison: A substance can cause harm only in amounts high enough to overwhelm the body’s defenses. But a major conceptual shift is underway, a leading expert said recently at AAAS, with much more attention being paid to low-dose chemical exposures and the impact they can have even many years later.

A mysterious and deadly cancer that has been plaguing populations of Tasmanian devils for more than a decade must have abandoned its original host long ago, essentially becoming a parasite of the devils, researchers say.