“There is limited evidence from past outbreaks that suggests there probably are quite a few people who get exposed, who get infected, without ever developing symptoms and without ever developing illness but they develop immunity,” said Lauren Ancel Meyers, a professor in the Department of Integrative Biology at The University of Texas at Austin.

In other words, Ebola might be acting like its own vaccine.

Meyers hasn’t tested any blood herself, but she and her team went through published reports about studies done on Ebola patients. In some outbreaks, up to half of people at high risk of infection who were watched never got sick, but they had antibodies against Ebola in their blood.