Following infection with HIV-1, the rate of clinical disease progression varies between individuals. Factors such as host susceptibility, genetics and immune function, health care and co-infections as well as viral genetic variability may affect the rate of progression to the point of needing to take medication in order not to develop AIDS. Rapid progressors A small percentage of HIV-infected individuals rapidly progress to AIDS if they fail to take the medication within four years after primary HIV-infection and are termed Rapid Progressors (RP). Indeed, some individuals have been known to progress to AIDS and death within a year after primo-infection. Rapid progression was originally thought to be continent specific, as some studies reported that disease progression is more rapid in Africa, but others have contested this view. Long term non-progressors Another subset of individuals who are persistently infected with HIV-1, but show no signs of disease progression for over 12 years and remain asymptomatic are classified as Long Term Non-Progressors (LTNP). In these individuals, it seems that HIV-infection has been halted with regard to disease progression over an extended period of time. However, the term LTNP is a misnomer as that progression towards AIDS can occur even after 15 years of stable infection. LTNP are not a homogeneous group regarding both viral load and specific immune responses against HIV-1. Some LTNPs are infected with HIV that inefficiently replicates whilst others are infected with HIV that is virally fit and replicates normally, but the infected individual has had a strong and broad set of HIV-specific humoral and cell-mediated responses that seems to delay the progression to AIDS. In some cohorts, individuals who experience signs of progression, but whose clinical and laboratory parameters remain stable over long periods of time, are classified as Long Term Survivors (LTS). Highly exposed persistently seronegative There is another, smaller percentage of individuals who have been recently identified. These are called Highly Exposed Persistently Seronegative (HEPS). This is a small group of individuals and has been observed only in a group of uninfected HIV-negative sex workers in Kenya and in The Gambia. When these individuals' PBMCs are stimulated with HIV-1 peptides, they have lymphoproliferative activity and have HIV-1 specific CD8+ CTL activity suggesting that transient infection may have occurred. This does not occur in unexposed individuals. What is interesting, is that the CTL epitope specificity differs between HEPS and HIV positive individuals, and in HEPS, the maintenance