Lassa fever is an acute viral, hemorrhagic (bleeding) disease mainly transmitted to humans by rats and/or food or household items contaminated with rodent urine or feces. Human-to-human transmission of Lassa fever can also occur. Lassa fever is endemic (common) in areas of West Africa.

Most individuals (80%) who are infected with the Lassa virus do not have any symptoms or only mild symptoms. Symptoms of Lassa fever begin 1-3 weeks after infection. Mild symptoms of Lassa fever may include low fever, headache, sore throat, cough, weakness, muscle pain, and malaise. Symptoms of more severe cases of Lassa fever may include facial swelling; hair loss; gait disorder; hearing loss; encephalitis (brain inflammation); nausea and vomiting; diarrhea; fluid in the lungs; respiratory distress; tremors; chest, back, or abdominal pain; miscarriage; hemorrhaging in the eyes, nose, gums, or gastrointestinal tract; low blood pressure; shock; seizures; coma; multi-organ failure; and death. The most common complication of Lassa fever is either temporary or permanent deafness.

Treatment for Lassa fever includes early supportive care and symptomatic treatment with adequate hydration, oxygen, blood pressure maintenance, and the antiviral drug, Ribavirin.