Fear of things that might actually hurt us, like the flu or smoking, is understandable and healthy. It’s the phobia of things—snakes, sharks, the youth—that pose virtually no threat at all that’s more puzzling. (The last shark attack death in the continental U.S. was in 2012. Meanwhile, 30,000 people die in car accidents every year). Even in 2013, people thought more frequently about the possibility of a terror attack in the U.S. than they did about the prospect of their own hospitalization.

An inability to process these kinds of odds ratios is one reason why some people experience irrational, sometimes crippling, unease. In an interview with New York magazine about why Ebola is sparking mass hypochondria in the U.S., Catherine Belling, an associate professor at Northwestern University’s Feinberg School of Medicine, chalked it up to a reasonable fear (you might get Ebola if you accidentally touch the bodily fluids of someone who has it) getting distorted by bad logic (you might get Ebola if you accidentally touch anyone, ever.)

“What hypochondria is, then, is the inability to put that very rational fear into context, where you can continue to function normally rather than being paralyzed by it,” she said. “When someone who is in, for example, New York, won’t leave their apartment because they’re afraid of getting Ebola. They’re incapable of recognizing that there’s truly an incredibly minuscule likelihood of getting Ebola.”