None of the cases were confirmed in the lab. And by the time researchers reached people, many had a hard time remembering what had happened months ago. But one thing is now clear, Rollin says: The entire outbreak in West Africa likely started with only one person, who caught the virus from a sick animal. Maybe a bat. Or another animal that had been infected by a bat. "So we think that there was only one introduction [from animals] and then from that it went from human to human to human to human," Rollin says. Scientists figured this out in two ways. First, every person they know who has been infected with Ebola, so far, has had contact with somebody else with the disease.

"It looks like they're either the same or very related," Rollin says. That match also suggests that there aren't hundreds of bat colonies swarming around West Africa infecting people. Instead there was just one source of the virus that infected a person way back in December, or possibly earlier.

 If that's the case, it means people in Guinea were getting Ebola and didn't realize it. That seems a bit surprising. How can you have Ebola and not realize it? There's actually a good reason. Another disease in West Africa has symptoms similar to Ebola. It's called Lassa Fever. It can even cause bleeding in the eyes, gums and nose. "Around five years ago, I was doing a study with a colleague from the U.S. Army, where we were looking at patients admitted for suspected Lassa fever, [but] who tested negative for Lassa," says Joseph Fair, an infectious disease doctor who works in Sierra Leone. 

"We always find a link with someone else who had been sick," Rollin says. "You always find this chain," he added. So the virus isn't just popping up seemingly randomly. Then there's the DNA evidence. Scientists have analyzed the genetic hot rolled steel coil of the virus in Sierra Leone and in Guinea.