Lassa fever, also known as Lassa hemorrhagic fever, is a type of viral hemorrhagic fever caused by the Lassa virus.^[1] Many of those infected by the virus do not develop symptoms.^[1] When symptoms occur they typically include fever, weakness, headaches, vomiting, and muscle pains.^[1] Less commonly there may be bleeding from the mouth or gastrointestinal tract.^[1] The risk of death once infected is about one percent and frequently occurs within two weeks of the onset of symptoms.^[1] Of those who survive, about a quarter have hearing loss, which improves within three months in about half of these cases.^[1][3][4]

The disease is usually initially spread to people via contact with the urine or feces of an infected multimammate mouse.^[1] Spread can then occur via direct contact between people.^[1][5] Diagnosis based on symptoms is difficult.^[1] Confirmation is by laboratory testing to detect the virus's RNA, antibodies for the virus, or the virus itself in cell culture.^[1] Other conditions that may present similarly include Ebola, malaria, typhoid fever, and yellow fever.^[1] The Lassa virus is a member of the Arenaviridae family of viruses.^[1]

Nigeria Centre for Disease Control reports 214 Lassa fever cases, 39 deaths in 2025, which increased compared to that reported for the same period in 2024.^[66]

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The Lassa virus is one of several viruses identified by WHO as a likely cause of a future epidemic. They therefore list it for urgent research and development to develop new diagnostic tests, vaccines, and medicines.^[70][71]

In 2007, SIGA Technologies, studied a medication in guinea pig with Lassa fever.^[72] Work on a vaccine is continuing, with multiple approaches showing positive results in animal trials.^[73]