One of the researchers, Gene Richardson, whose specialties include infectious disease and anthropology, told the AP that Ebola, like most other viruses, could infect people but would show few symptoms.

“It provides important evidence on that front. It also means a significant portion of transmission events may have gone undetected during the outbreak,” he said.

“This shows there was a lot more human-to-human transmission than we thought.”

Ebola is spread through close contact with the bodily fluids of an infected person, including blood, vomit and diarrhea. Since many patients become violently ill, family members and medical professionals who care for them face a high risk of infection.

Researchers recruited 187 men, women and children from Sukudu, a rural village of about 900 people in eastern Sierra Leone for the study.

Participants had likely been exposed to Ebola through a family member or by using a public toilet shared by a person who had the disease.

“Of these, 14 were found to be carrying antibodies to Ebola, suggesting they had been infected at some point, though they had not been included in the original count,” said the study in PLOS Neglected Tropical Diseases.

Twelve said they had no symptoms, while two recalled having had a fever at the time of the outbreak.

The village was one of three major hot spots in the Kono district during the Ebola crisis in 2014 and 2015. Thirty four people in the village were infected with Ebola and 28 died.

Given the 14 previously unreported cases, researchers calculated that about one quarter of people in the village who became ill with Ebola had no symptoms or very minor ones.

Ebola is known to linger in semen for months after the initial infection, though it remains unclear whether people who never had any symptoms could spread Ebola to a partner.

The senior author on the study was Paul Farmer, a Harvard University professor and director of Partners In Health.