Ebola doesn't always show itself through fever, vomiting, and diarrhea. A new PLOS study finds that some people with the virus show mild or no symptoms at all—a potential concern for preventing its spread.

Researchers who visited the village of Sukudu in Sierra Leone, a hot spot in the recent West African Ebola outbreak, found up to 25 percent of Ebola infections "may have been minimally symptomatic," meaning "a significant portion of Ebola transmission events may have gone undetected" in Guinea, Sierra Leone, and Liberia, NBC News reports.

At least 28,000 people are known to have been infected during the epidemic, at least 11,000 of whom died. In Sukudu, there were 34 known Ebola cases, including 28 deaths among 900 residents, reports Gizmodo.

A year after the worst of the outbreak was over, Stanford University researchers tested the blood of 187 people in Sukudu who'd lived with or shared a toilet with someone infected with Ebola, per a release.

They found 14 had Ebola antibodies, suggesting they'd previously been infected, while 12 said they didn't recall feeling sick or having a fever. Believing them to be truthful as health workers kept an eye on villagers during the outbreak, researchers now plan to visit other villages "so we can begin to answer the question of what was the true burden of disease," the study author says.

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"We expect to find a lot more undocumented survivors." It's not clear if asymptomatic patients are contagious. "They were not passing it along in the usual way, through vomiting or diarrhea," he says.

"It's unclear if they can pass it along sexually."

This article originally appeared on Newser: You Could Have Ebola and Not Even Know It