Whooping cough. Measles. These diseases, once thought almost gone, are creeping back into schools and hospitals around the country. The reason? Parents are refusing to have their children vaccinated, because they’re afraid that the shots can cause autism.

This ideas stems from a 1998 study in the medical journal The Lancet, in which British doctor Andrew Wakefield suggested the MMR (measles, mumps, and rubella) vaccine may be linked to autism.

The journal has since withdrawn the study, Wakefield lost his doctor’s license, and the British Medical Journal declared it fraud. But that hasn’t stopped celebrities such as Jenny McCarthy from declaring that there’s a link.

The problem comes from our idea of truth, says Seth Mnookin, author of the newly released The Panic Virus: A True Story of Medicine, Science and Fear. Who’s easier to believe? Scientists and doctors you don’t know, or your neighbor with the autistic son, who said his symptoms started right after a booster shot?

“When it comes to decisions around emotionally charged topics, logic often takes a back seat to what are called cognitive biases — essentially a set of unconscious mechanisms that convince us that it is our feelings about a situation and not the facts that represent the truth,” Mnookin said in a recent essay in The Atlantic.