There are two major types of the human immunodeficiency virus. HIV-1, which was discovered first, is the most widespread type worldwide. HIV-2 is more than 55% genetically different from HIV-1.
HIV-2 is most common in western Africa and is becoming more common in India, although numbers there are still relatively small. Small numbers of cases have also been seen in Portugal, France, other European countries including the UK and the Americas, largely in individuals of west African origin or their sexual partners.
Each major type of virus can be further broken down into groups, which themselves can be subdivided into clades or subtypes. HIV-1 comprises groups M (main), O (outlier), and N (non-M or O). There are two main HIV-2 subtypes, A and B.
Due to the genetic difference, HIV-1 and HIV-2 antigens are distinct enough that if a diagnostic test is developed only to detect HIV-1, it will not reliably detect HIV-2. However, tests which are sensitive to both types of virus are available. It is important to use a test that is sensitive to HIV-2, especially when testing someone who has lived in a country where HIV-2 is common or has had a sexual partner from one of those countries. Moreover, if a person has clinical signs of HIV infection (e.g. recurrent opportunistic infections) but does not test positive to a test which is sensitive only to HIV-1, then testing specifically for HIV-2 would be appropriate.