What is Lassa fever?

Lassa fever is an acute viral haemorrhagic illness caused by Lassa mammarenavirus (Arenaviridae family). It is endemic in West Africa — primarily Nigeria, Sierra Leone, Liberia, and Guinea — and was first identified in 1969 in Lassa, Nigeria. The disease is a zoonosis: its primary reservoir is the multimammate rat (Mastomys natalensis). Approximately 80% of infections are mild or asymptomatic; the remaining 20% can progress to severe disease involving haemorrhagic manifestations and multi-organ failure.