The Ebola virus is mutating "quite a lot," according to scientists tracking the outbreak in West Africa—but the tough part is telling just what it is mutating into. It is important to track genetic changes to "keep up with our enemy," a geneticist at France's Pasteur Institute, which is analyzing blood samples from hundreds of patients, tells the BBC. He says researchers have now seen several cases of people who have the virus but don't show any symptoms. "These people may be the people who can spread the virus better, but we still don't know that yet," he says. "A virus can change itself to less deadly, but more contagious, and that's something we are afraid of."
Like HIV and flu viruses, Ebola is an RNA virus capable of mutating quickly, the BBC notes, but while scientists fear it could mutate enough to become airborne, an earlier World Health Organization study stressed that "gene mutations may not have any impact on how the virus responds to drugs or behaves in human populations." The WHO says that in Guinea, Liberia, and Sierra Leone, the three countries most severely affected by the outbreak, a total of 99 new cases were confirmed last week, which is the lowest total since June, USA Today reports. The Pasteur Institute is working on two Ebola vaccines that it hopes to have ready for human trials by the end of this year, including a modified measles vaccine that would protect against both diseases. (This might be the origin of the Ebola outbreak.)