“Once trust starts to erode, the next time they tell you not to worry — you worry”

For example, most people appreciate that a chance of infection of one in a 100 million is near zero. But if a friend says he knows an infected person, then our instinctive risk-assessment system is much more likely to focus on the numerator than on the denominator. Am I the one in 100 million? Me?

“The system often flips from one extreme to another, from ignoring risks altogether and then overreacting,” said George Loewenstein, a professor of economics and psychology at Carnegie Mellon University.

The Ebola outbreak has many of the elements that could quickly stoke instinctual panic, experts note. It is invisible and deadly, a point graphically communicated in nonstop pictures and videos from Africa. Like SARS, and more recently MERS, Middle Eastern Respiratory Syndrome, and bird flu, it is a strange, exotic threat, and there is little that can be done personally to limit it.

These elements are precisely those that are most likely to cause contagious anxiety, researchers have found.