Throughout West Africa as many as 300,000 people are infected annually with Lassa virus, resulting in approximately 5,000 deaths. Most commonly, contact with infected rodent hosts (Mastomys natalensis), or ingestion / inhalation of virus laden particles is the source of human infection. However, person-to-person transmission is also well documented and can result in outbreaks, especially in nosocomial settings. Infection in pregnancy, especially the third trimester, is particularly severe with maternal mortality rates estimated at 20% and fetal mortality rates nearing 100%. A survey of rodents captured in the village of Soromba (rural commune of Sibirila, district of Bougouni, Mali) found that 25% of Mastomys natalensis had evidence of Lassa virus infection.