Lassa fever refers to systemic infection with the Lassa virus from the genus Arenavirus of the Arenaviridae family. This genus is divided into two groups based on the location of the rodent reservoir: Old World (Eastern hemisphere) or New World (Western hemisphere). The diseases all share a common feature of being transmitted to humans by contact with native rodent urine or excreta.

The Old World Arenaviridae include Lassa virus (from West Africa) and Lymphocytic choriomeningitis virus (found in Europe, Asia, and the Americas) as well as the newly discovered Lujo virus (isolated in South Africa; patients develop Lujo-associated hemorrhagic fever, which has a high case fatality rate [80%], after inhalation of dust containing the virus, by ingestion of contaminated foods, or by inoculation with infected body materials). Lassa virus is enzootic in the West African multimammate mouse Mastomys natalensis. Outbreaks have been reported in several countries of West Africa.

Lassa fever infections can have widely varied clinical presentations. Most infections are asymptomatic and mild symptomatic cases may not be distinguished from common respiratory or gastrointestinal infections. The incubation period is 7-14 days and initial symptoms include headache, body aches, cough, nausea, sore throat, and fever. The fever is usually high and may be intermittent or constant. Five days after the onset of fever, hemorrhagic phenomenon may be observed and are associated with dehydration, shock, and cardiovascular collapse. Edema of the face and neck is common, which can cause stridor, respiratory distress, and central cyanosis. Encephalopathy and seizures are common.

The mortality rate is 15% in hospitalized patients and fetal loss is common in infected pregnant patients. Hair loss and loss of coordination is seen during convalescence. Deafness is seen in 25% of survivors.

Vaccines are under development, but are not widely available yet.