Some people infected with the usually deadly Ebola virus show no symptoms of the illness at all, researchers have confirmed, most likely because of a rapid immune response controls the virus in these people.

The Ebola virus, which often kills 70% or more of those infected, and causes severe symptoms; patients quickly develop high fever, aches, fatigue and may vomit blood or have bloody diarrhea. It is widely believed that people only contract Ebola if they come in contact with blood or secretions from an infected person. Most victims die within 5 to 7 days.

Researchers studied 11 people with symptomless Ebola infections and found that all of them had antibodies to the virus in their blood. Seven of the 11 were also found to have genetic material from the Ebola virus in their blood, according to a highly sensitive gene amplification technique called PCR.

Until now it was thought that symptomless Ebola infections were possible but it had not been proven. Scientists have known, for instance, that the illness can be mild in some people. This study shows not only that symptomless Ebola occurs, but also that it is likely due to a particularly strong immune response.

The Lancet June 30, 2000;355:2178-2179, 2210-2215.