HTLV-II is thought to have jumped from monkeys to humans 15,000 to 30,000 years ago in the Americas. For up to 2000 generations, the virus was relatively benign and inactive, most common in Pygmy and American Indian tribes, transmitted from mother to child via breast-feeding. In the past 30 years, however, the virus has spread into a wider population through contaminated blood in shared needles. The virus is now in the blood of between 5 and 10 percent of drug users in USA, Europe and Asia. In drug users, the rate of evolution of the viral genetic sequence is believed to be 300 times faster than in endemic populations, probably because with unrelated people it is required to mutate to survive. Each new infection forces the virus to evolve. HTLV-I, can cause leukaemia and neurological disorders, and now severe neurological disease has been picked up in several drug users infected by HTLV-II, although it is still unclear whether this is linked instead to their known infection with HIV.
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