Why is it that we assume that a person who is infected with ebola and survives is no longer infected ?
Because a false sense of security makes some people feel better.
It's called immunity. When someone receives a vaccination or survives an active infection with most organisms, they build up antibodies that protect them from a future infection. Transfusions from compatible donors who have survived an Ebola infection have been successfully used to treat some Ebola patients. I believe that is the only "drug" that Dr. Brantley received and he has since donated his blood to help save the lives of others.