Like many other viruses HIV also has the ability to mutate over time. Since the first discovery in 1980s, different types of HIV strains have been found and these strains have been classified into two main types called HIV-1 and HIV-2.
On a genetic level HIV-2 is more than 55% different from HIV-1, leading to functional differences. HIV-1, which was the first discovered type, is the most widespread type accounting for 95% of all infections worldwide. HIV-2 is more commonly found in Western Africa, it is known to be less infectious and progresses slower than HIV-1.
The functional and genetical differences between the two also effects the testing and treatment methods. Some HIV drugs do not work effectively on HIV-2 and even though there are tests which are sensitive to detect both types, due to the genetical difference a test which is developed to detect antigens of one main type cannot always detect the other. This has to be taken into consideration when testing for a possible HIV-2 infection. 1
Additionally, each main type can also be broken down to subtypes. HIV-1 is classified into four different groups: M, N, O and P. Group M stand for the ‘Major’ group and is responsible for most of the global HIV epidemic. Under the group M there are at least nine different subtypes. These are subtypes A, B, C, D, F, G, H, J and K. Subtype B is more prevalent in America, Western Europe and Australasia. It accounts for 12% of all global infections and most of the clinical trials are conducted in the areas where subtype B is more prevalent. Although subtype C, which is more prominent in South Africa and India, represents nearly 50% of all HIV infections globally, less research has been conducted on this subtype.2
HIV-2 is also classified into eight different groups: A, B, C, D, E, F, G, H. Only groups A and B are pandemic and while both groups are mainly found in Western Africa, group A is also found in Angola,Mozambique,Brazil,India,Europe, and in theUS. 3
There are also Circulating Recombinant Forms (CRFs), which are types that mutate to from recombination of different subtypes, there are currently at least 89 CRFs under HIV-1.1
Footnotes
1 https://www.avert.org/professionals/hiv-science/types-strains
2 http://www.aidsmap.com/HIV-1-subtypes/page/1322996/
3 http://aidsinfo.nih.gov/guidelines