Influenza H5N1 virus frightens many because of the widely quoted case fatality ratio of >50%, which is based on the number of deaths among the fewer than 600 cases confirmed by the World Health Organization. Such fear is misguided, because it is likely that the fatality ratio is far lower. For example, studies of >7,000 healthy individuals have revealed that about 0.5% of them carry antibodies to the virus in their blood, indicating that mild or asymptomatic infections do occur. T-lymphocytes that recognize influenza H5N1 virus have now been detected in a high-risk cohort of individuals in Vietnam, providing additional evidence for asymptomatic human infections.
