AIDS (Acquired Immunodeficiency syndrome) is an infectious disease that attacks the immune system of the body. HIV (Human Immuno Deficiency virus) is a member of a group of viruses called retrovirus, that causes AIDS. AIDS virus enters into the body via the sexual organs or infected blood or from mother to baby and then spread to lymph nodes all over the body. So, sexual transmission is not the only way for AIDS virus transmission.
There can be no surety about who was the first person infected with AIDs virus and how was he/she infected. But, this can be speculated. It is though by some , that AIDs virus first transmitted from chimpanzees to human by the use of polio vaccine which are made using chimps.