that HIV is closely related to similar immunodeficiency viruses, which infect monkeys. So it has been suggested that HIV evolved from viruses that originally infected monkeys in Africa and was somehow transmitted to humans from there.
             Researchers believe that the HIV infection became widespread after social changes took place in Africa during the 1960's and 1970's. Large numbers of people moved from rural areas to cities, resulting in crowding, unemployment, and prostitution. These conditions brought about an increase in cases of sexually transmitted diseases, including AIDS. HIV may have been introduced into industrialized nations several times before transmission was
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