Fever, Lassa: An acute viral infection found in the tropics, especially in West Africa. Lassa fever is caused by a single-stranded RNA virus that is animal borne (zoonotic). Lassa fever can cause a potentially fatal illness, is highly contagious, and can rapidly spread. The reservoir, or host, of the Lassa virus is a rodent known as the 'multimammate rat.' People can become infected by eating this infected rat or by eating food contaminated by the rat's excretions. Person-to-person transmission also occurs via direct contact, contamination of skin breaks with infected blood, and aerosol spreads (virus particles moving through the air). The first symptoms typically occur 1 to 3 weeks after the patient comes into contact with the virus and may include high fever, sore throat, cough, eye inflammation (conjunctivitis), facial swelling, pain behind the breastbone, back pain, abdominal pain, vomiting, diarrhea, and general weakness that lasts for several days. The antiviral drug ribavirin is used for treatment along with supportive care.