This post originally appeared on NPR.

The Ebola virus doesn’t always make people incredibly sick and some people who are infected don’t even know they have it, according to research published Tuesday in PLOS Neglected Tropical Diseases.

“We are finding more and more that there [are patients with] minimally symptomatic Ebola infections,” says Dr. Gene Richardson, lead author of the study. “And why wouldn’t there be? Just about every other virus that causes infections in humans has a spectrum. Here there is a spectrum of illness from minimally symptomatic to death.”

In 2014 Ebola hit the village of Sukudu in eastern Sierra Leone. Officially 34 cases were diagnosed among roughly 800 residents from November of that year until February 2015. Twenty-eight people died and six patients survived. Richardson and his colleagues wondered if more people in the village could have been infected, but hadn’t checked in at the local treatment center.

Richardson, who teaches medicine at Brigham and Women’s hospital in Boston and is pursuing a Ph.D. in anthropology at Stanford, set out to identify villagers who may have been been unaware that they were infected by the virus.

“During that time [the outbreak] all the houses that had a case were quarantined. All of the houses that shared a public latrine with those houses were quarantined and all the neighbors were quarantined,” Richardson says. “We went back and drew blood from everybody that was quarantined.”

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