A new study raises the terrifying possibility that the Ebola virus outbreak in West Africa may have been larger than anyone realized.

During the height of the crisis, the military quarantined whole villages, and so many people were dying very painful and very visible deaths that there wasn't enough time to bury them. At the time, the idea that there might be a significant number of people infected with the virus but with minimal or even no symptoms was not something that public health officials had much time to think about. This may have been an important oversight.

Researchers writing in PLOS Neglected Tropical Diseases this week reported that they had found evidence of a cluster of people infected with a type of “walking” or invisible Ebola.

Their work focused on one village in the diamond-rich district of Kono in eastern Sierra Leone. From October 2015 to January 2016, they collected blood samples from 187 people from homes that had been quarantined because of possible exposure to an Ebola patient. The scientists found that 14 individuals who had not been previously identified as having Ebola were infected. Only two of them said they had had a fever during the time when the virus was spreading, but the other 12 said they didn't notice any signs or symptoms.

While this study is very small relative to the 28,000 cases of Ebola that have been reported during the outbreak, it raises important questions about how the virus is transmitted and about what defines a survivor, and it may influence how vaccines should be developed and tested.

“The findings provide further evidence that Ebola, like many other viral infections, presents with a spectrum of clinical manifestations, including minimally symptomatic infection,” Eugene Richardson from Stanford University and his colleagues wrote. “These data also suggest that a significant portion of Ebola transmission events may have gone undetected during the outbreak.”

Read more: