Lassa virus' natural reservoir is Mastomys natalensis, a rodent prevalent throughout Sub-Saharan Africa. Many Mastomys species live primarily in and around homes in this region. Most human infection is thought to be through aerosol transmission or contact with Mastomys excretions and blood resulting from capture and killing for consumption.
Of all of the diseases caused by arenaviruses, Lassa fever has the greatest health impact. Severe Lassa virus infections occur in adults of both sexes as well as children. The annual number of cases of Lassa fever is estimated at 20,000-40,000, with several thousand fatalities. Overall mortality among hospital cases is approximately 20%, usually following cardiovascular collapse. In areas where ribavirin (see link: Vaccination) and careful medical management are unavailable to ameliorate the health impact, untreated Lassa fever can be fatal to 60% of its victims. Lassa virus also causes unusually high fetal mortality, with studies showing that fetal loss can be 92% if the mother is infected early in pregnancy, and 75% in the last trimester. The pregnant woman also has an increased risk of death from Lassa fever.
Infection with Lassa virus leads to the gradual onset on fever and malaise after an incubation period of about 10 days. As the course of the disease progresses, severe prostration and systemic illness, with changes in vascular permeability and vasoregulation, can occur. Bleeding is seen in less than a third of those infected, but usually signals an unfavorable prognosis. These most severe instances are known as Lassa hemorrhagic fever. Lassa fever is the most exported of the viral hemorrhagic fevers, with cases treated in the United Kingdom, the United States, Japan, Israel, and The Netherlands.
Click here for more on Lassa Virus
Another source for Lassa and other arenavirus information
Fields, Virology, 1996. pgs. 1525-1538.
White and Fenner, Medical Virology, San Diego: Academic Press. 1994. pgs. 505-506.