- Lassa fever is very similar to other diseases that cause fever, such as malaria and yellow fever. People get Lassa fever from mice. A person can get Lassa fever from mice urine and droppings. When a group of researchers tested the mice in Soromba, Mali, they found that many were infected with Lassa fever. Lassa fever may also be passed through body fluids (like blood or semen) of people infected with Lassa fever. Researchers want to study this disease to help the government better protect the health of people in Sibirila.