Lassa virus, an arenavirus first isolated in 1969 in Jos, Nigeria (1), is the cause of Lassa fever, an acute viral illness that affects 100,000 to 300,000 persons per year based on 1970s estimates (2). Lassa fever is endemic in regions of West Africa, including Guinea, Liberia, Nigeria, and Sierra Leone, but cases have been exported to other countries by infected travelers. The natural reservoir for Lassa virus is the African soft-furred rat (Mastomys natalensis), which may be found throughout West Africa. The virus is transmitted to humans via direct contact with or the inhalation or ingestion of infected rat excreta or person to person via contact with infected body secretions (3).
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