• A number of strains of HIV-2 have been identified, classified into four clades (A, B, C, D) which are no more closely related to each other than they are to different strains of an SIV (simian immunodeficiency virus) found in wild sooty mangabey monkeys in West Africa. It therefore appears that the different clades represent separate transmissions of the virus at some time in the past from sooty mangabeys to people

• HIV-2 infection appears to be less pathogenic than HIV-1. It is much more difficult to isolate HIV-2 from the blood of infected individuals than in cases of HIV-1 infection

• Immunodeficiency caused by HIV-2 infection takes much longer to develop. Individuals infected with HIV-2 are asymptomatic for much longer, and may not develop high levels of virus in their blood for fifteen to twenty years after infection, by which time they may be much less sexually active or past child-bearing age

• HIV-2 is not easily transmitted during the lengthy asymptomatic phase of infection due to the very low levels of virus found during this time, which may explain why the virus is less widespread. Studying sex workers in Senegal, researchers found that whilst HIV-1 spread very rapidly (the number of new cases of HIV-1 was greater during each year of the study than the preceding year), HIV-2 spread much more slowly (the number of new cases each year was almost identical). Researchers estimated that it would take 5 years for the number of HIV-1 cases to double, but 31 years for the number of HIV-2 cases to double (Kanki)

• HIV-2 is less easily transmitted from mother to baby, perhaps because of much lower viral load in HIV-2 infected mothers (Gayle).

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