HIV usually refers to the HIV-1 strain; but another strain, known as HIV-2, was first described in 1986 in West Africa. It is associated with the same opportunistic illnesses that result from HIV-1 infection; however, HIV-2 appears to have a milder and slower effect on the immune system. The mode of transmission is the same, although HIV-2 seems to be less transmissible from mother to child than HIV-1.