Lassa virus particles. Transmission electron micrograph (TEM) of Lassa virus particles (virions, green) amongst cell debris. This Arenavirus is the cause of Lassa fever. On the particles' surfaces are proteins (purple) that enable the virus to attach to host cells. Lassa fever occurs in West Africa. Rats harbour the virus, which is spread by inhaling rat urine, or coughed droplets from infected patients. High fever, headache and severe muscular pain result, and death may occur due to heart or kidney failure.