Genetics
Seven genotypes are recognised and about 20 million people are infected. 3-8 million are in Africa. The most widespread genotype is type A. Type B has only been isolated from Central Africa to date (2012). Type C is prevalent in Melanesia. Type D is also found in Central Africa.
The virus appears to have evolved from simian T-cell leukemia virus type 1.
Two subtypes are found in Japan: a transcontinental subgroup and a Japanese subgroup.
Read more about this topic: Human T-lymphotropic Virus 1