Lassa fever virus is a member of the arenavirus family ( 1). The name stems from arenosus -- Latin for sandy -- because of the characteristic fine granules seen by electron microscopy. Arenaviruses cause persistent infection in the host, that is, long-term infection that does not directly kill. Persistent infection, in general, does little harm to its animal host, because the two have evolved a near-symbiotic relationship, usually over the host's lifespan. The natural host of an arenavirus is often restricted to a single kind of rodent. The rodent host carries these viruses in its blood and passes them in its urine. It is by contact with such excretions from the rodent that humans become infected. Although no chronic or persistent arenavirus infections have been found in humans, Lassa fever virus has been isolated from the urine of patients as late as one month after the onset of acute disease. There are no known insects that transmit this disease. Consequently, spread to humans occurs only when humans come in close contact with the infected rodents in their natural habitat.