​Lassa virus, a member of the Arenaviridae family, can cause viral hemorrhagic fever (VHF).  The virus is endemic to West Africa.   It is usually transmitted by the Mastomys natalensis (multi-mammate rat).  The virus is spread by contact with excretions or materials contaminated with the excretions of an infected rodent or inhalation of contaminated dust and bite wounds.  Infection can also occur after exposure to virus in the blood, tissue, secretions and excretions of a Lassa virus infected individuals. Symptoms of a Lassa infection usually start within 1-3 weeks after contact with the virus.  Because of the non-specific clinical features, early diagnosis is difficult.  Most Lassa virus infections are mild and go undiagnosed.  However, approximately 20% of infected individuals develop serious symptoms that include a high fever, hemorrhaging, facial swelling, retrosternal chest pain, and neurological complications such as tremors and encephalitis.  The most common complication of Lassa fever is hearing loss. Because the symptoms of Lassa fever are so varied and nonspecific, clinical diagnosis is often difficult. Lassa fever is also associated with occasional epidemics, during which the case-fatality rate can reach 50% in hospitalized patients.

For more information:    https://www.cdc.gov/vhf/abroad/assessing-vhf-returning-traveler.html​