Ebola immunizing as well as killing?

A sharp drop in new Ebola infections in West Africa is prompting scientists to wonder whether the virus may be immunizing some people at the same time as killing their neighbors.

So-called "asymptomatic" Ebola cases - in which someone is exposed to the virus and develops antibodies but does not get sick or suffer symptoms - are disputed among scientists, with some saying their existence is little more than a pipe dream.

Yet if, as some studies suggest, such cases do occur in epidemics of the deadly disease, they may be a key to ending outbreaks quickly by protecting those who can keep the infection at bay.