Q: I know what a placebo is....

Q: I know what a placebo is. What is a nocebo?

A: A placebo relieves symptoms of illness by creating an expectation of improvement, while a nocebo does harm by creating the opposite expectation, according to epidemiologist Robert A. Hahn of the Centers for Disease Control and Prevention. Nocebos are not used in medical research, but nocebo effects are quite common. A pessimistic person, for example, is more likely to develop symptoms of some illness. Some people witnessing an epidemic of an infectious agent, such as influenza, develop symptoms themselves, even though they have not been infected. This phenomenon is sometimes called mass hysteria or, in the workplace, assembly line hysteria.