This transmission electron micrograph (TEM) depicted numbers of Lassa virus virions adjacent to some cell debris. The virus, a member of the virus family Arenaviridae
, is a single-stranded RNA virus, and is zoonotic, or animal-borne that can be transmitted to humans. The illness, which occurs in West Africa, was discovered in 1969 when two missionary nurses died in Nigeria, West Africa.
In areas of Africa where the disease is endemic (that is, constantly present), Lassa fever is a significant cause of morbidity and mortality. While Lassa fever is mild or has no observable symptoms in about 80% of people infected with the virus, the remaining 20% have a severe multisystem disease. Lassa fever is also associated with occasional epidemics, during which the case-fatality rate can reach 50%.