This post originally appeared on The Wall Street Journal.

Researchers identified a number of people who were infected with deadly virus but didn’t report being sick.

ATLANTA—More than two years after the explosive Ebola epidemic in West Africa, researchers have identified a number of people who were infected with the deadly virus but didn’t report being sick, suggesting the outbreak may have been more widespread than currently believed.

Twelve of 48 people who were infected with Ebola in a village in eastern Sierra Leone reported that they had experienced no symptoms of the disease, though blood tests found antibodies indicating infection, according to the researchers from Stanford University, the Boston-based charity Partners in Health, and other institutions.

The tests found that as many as 25% of those infected in Suduku, a hotspot for transmission, had minimal or no symptoms, the researchers concluded. Already, West Africa’s Ebola epidemic was by far the largest in history to be caused by the virus, with 28,646 reported cases in 10 countries, including 11,323 deaths.

Ebola is one of the deadliest viruses known to man, but public-health experts have long suspected that the virus produces a wide range of symptoms—killing some people while leaving others who are infected without any illness.

“It tells us there was a lot more human-to-human transmissions than we thought,” saidGene Richardson, lead author of the study, a Ph.D. candidate at Stanford and an infectious-diseases consultant at Brigham and Women’s Hospital in Boston. “You have to prevent every instance of human-to-human transmission, because you don’t know which ones are going to be symptomatic.”

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