Lassa fever

Target animals: ,

Pathogen type:

About Lassa fever

Lassa fever is an acute viral haemorrhagic illness in humans that is caused by Lassa virus. The virus is zoonotic, with the primary reservoir host being Mastomys rodents. Lassa fever is endemic across many West African countries. The overall case-fatality rate is low (1%), but infection is common, resulting in thousands of deaths every year. The case-fatality rate among patients hospitalised with severe cases of Lassa fever is 15%.

About 80% of people infected with Lassa virus have no symptoms. However,  20% of infections result in severe disease, where the virus affects several organs such as the liver, spleen and kidneys.

100,000 – 300,000

Number of infections of Lassa fever virus occurring annually.


Approximate number of deaths from Lassa fever every year.


Percentage of people hospitalised with Lassa fever annually in Sierra Leone and Liberia.