Lassa fever is an animal-borne, or zoonotic, acute viral illness spread by the common African rat. It is endemic in parts of West Africa including Sierra Leone, Liberia, Guinea and Nigeria. Neighboring countries are also at risk because the animal vector lives throughout the region.
The first documented case occurred in 1969. Lassa fever is named after the town in Nigeria where the first cases occurred.
About 100,000 to 300,000 infections of Lassa fever occur annually, with about 5,000 deaths. Surveillance for Lassa fever varies between locations so these estimates are crude. In some areas of Sierra Leone and Liberia, about 10-16% of people admitted to hospitals annually have Lassa fever. This shows the serious impact the disease has on the region.
The Lassa virus, shown here, causes Lassa fever.