Various investigations into the prevalence of HIV2 infection undertaken on samples of senegalese rural populations from Casamance, Eastern Senegal, the Thies Region and the Ferlo Region, show a gradient from south to north. Levels vary from 0.9% (western Casamance) to 0% (Ferlo). These results led to a detailed investigation being undertaken in a Casamance village, paralleled by questioning reference cases to define risks associated with HIV2 seropositivity. Prevalence in this village is 0.82%, and all age groups are affected. HIV2 disrupts the immune system before clinical symptoms appear. 10% of the seropositives can be classed as CDCIV.