Ask random folks on the street what the symptoms of Ebola are, and bleeding from every bodily orifice is the one that's likely to be stuck in their minds—probably because other symptoms (including fever, muscle pain, weakness, and headache, notes the CDC) aren't quite as visually graphic and disturbing. The bleeding "can take on a variety of shapes and forms, with the worst cases being people vomiting blood, having bloody diarrhea, bleeding from their nose and mouth and where the sun don't shine," Angela Rasmussen, a microbiology professor at the University of Washington, tells LiveScience. Yet not everyone who contracts the illness experiences "hemorrhagic syndrome," which typically sets in a day or two before death—only 30% to 50% of people afflicted will experience this type of bleeding. And just 18% of the people affected by the current outbreak in West Africa have experienced hemorrhaging, the CDC notes.
Doctors understand the basic underlying causes of the bleeding, if not why only some people develop it: Ebola can infect the liver, which then produces a blood-clotting protein that can "go into overdrive" and create clots that clog blood vessels; eventually the liver can't produce any more of these proteins, and blood vessel cells become inflamed and start to leak. Hemorrhaging symptoms typically show up between five to eight days into the illness, and mice in one of Rasmussen's studies that came down with the hemorrhagic syndrome died within seven to 10 days of contracting Ebola. She's hoping that further mice studies could offer clues about the possibility of genetic predisposal to the bleeding. Because not as many infected people experienced bleeding in the current West Africa outbreak, the virus may have initially escaped detection. "It was geographically outside of what we previously thought was the range for Zaire Ebola virus, and since without hemorrhage, it appears similar to malaria or typhoid," Rasmussen says. (Authorities in Spain are planning to euthanize a dog they fear may be carrying the virus.)