(NBC NEWS) The deadly Ebola virus that’s killed more than 600 people so far in West Africa may have been smoldering there for years and has almost certainly sickened people who thought they had something else, researchers say.

A new study of blood samples from people being treated for a serious, viral-like illness in years past in Guinea, Sierra Leone and Liberia suggests some of them could have been infected with Ebola. Now researchers are digging deeper to see if the virus has always been lurking there, just undetected..

It’s not a perfect test — the blood samples were several years old and had to be heat-treated so the researchers would be safe — but it’s a lead worth following up, said Randal Schoepp of the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) team that did the research.

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