A new study sheds light on another unfortunate aspect of West Africa’s recent Ebola pandemic.

Researchers from Stanford University and other institutions revisited a village in Sierra Leone called Sukudu to see if any cases of infection had evaded their detection. The scientists found evidence that the pathogen may be able to infect individuals without producing any of the typical symptoms linked to the disease like unexplained bleeding and headaches. The result shows that new methods need to be created to diagnose and contain the virus during an outbreak.

Sukudu, home to 900 residents, was a major hotspot for the outbreak in the Sierra Leone region. The area recorded 34 cases of Ebola during the epidemic including 28 deaths, according to Gizmodo.

The team took blood samples from 187 men, women, and children from the village who had probably been exposed to Ebola either through close contact with someone at home who has been infected or sharing a public toilet with an individual carrying the virus.

Results revealed 14 people were carrying an antibody to Ebola, which indicated they were infected at an indeterminate point. Furthermore, two people in this test group admitted they reported symptoms during the outbreak while the other 12 told investigators they had no symptoms during the active transmission period, per Stanford’s official announcement.

“The study corroborates previous evidence that Ebola is like most other viruses in that it causes a spectrum of manifestations, including minimally symptomatic infection,” said Dr. Gene Richardson, the lead researcher and Ph.D. candidate in Stanford’s anthropology department, in a statement. “It provides important evidence on that front. It also means a significant portion of transmission events may have gone undetected during the outbreak. This shows there was a lot more human-to-human transmission than we thought.”

Essentially, these preliminary findings suggest efforts to contain the virus were not entirely effective, but Richardson and his colleagues are working in other Sierra Leone-based villages where public health surveillance was poor so they can continue to verify the true number of people infected during the outbreak.

The study was published in the journal PLOS Neglected Tropical Diseases.