The purpose of this study is to determine the proportion of people who have been previously exposed to Lassa virus, as well as the one-year incidence of infection in three villages within the rural commune of Sibirila, district of Bougouni. We will obtain 2 fingerstick blood samples (approximately one year apart) from each of approximately 600 participants aged greater than or equal to 6 months. Participants will be passively followed for one year and asked to present to their local health center if they have a fever.