First reported in Lassa, Nigeria, and caused by an arenavirus transmitted by rodents or directly from an infected person. The incubation period is 3–21 days. The disease is characterised by headache, lethargy and severe muscular pains, and there is often a rash due to bleeding into the skin and mucous membranes. Sore throat is often present. It may carry a high mortality rate, particularly in pregnant women, and there is no specific treatment beyond intensive care support.