I think it is impossible to know the first cause of HIV because it wasn’t until the virus had progressed to AIDS that the problem was known. I also suspect that some people that didn’t receive medical care had AIDS before it was reported. I will, however, try to provide as many details and links showing evidence of the first case of AIDS.

HIV is believed to be a descendant of Simian Immunodeficiency Virus. There are strains of the virus that are close. The HIV-2 is closely related to the virus found in the sooty mangabey in West Africa. Other counter parts have been found in chimpanzees, but it has differences.

More about this can be found here:

http://www.avert.org/origin-aids-hiv.htm

As far as the time period of HIV or AIDS, that remains unknown, but it seems to be believed that people did contact the virus in the 1970s. Then a more wide spread of it happened in the 1980s.

It was thought at first to be Kaposi’s Scarcoma
–quote–

Kaposi's Sarcoma (KS) was a rare form of relatively benign cancer that tended to occur in older people.
March 1981 at least eight cases of a more aggressive form of KS had occurred amongst young gay men in New York.2
–end quote–

http://www.avert.org/aids-history-86.htm

It was in 1983 when women started coming up with the disease many people see this was not a “gay” disease.

The most common ways to get contract HIV include:

* having sexual intercourse with an infected partner.
* injecting drugs using a needle or syringe that has been used by someone who is infected.
* as a baby of an infected mother, during pregnancy, labor or delivery, or through breastfeeding.

http://www.avert.org/howcan.htm

http://www.webmd.com/hiv-aids/guide/sexual-health-aids
http://www.mayoclinic.com/health/hiv-aids/DS00005

The first recognized case of HIV was in 1980s in USA. In New York and California, many gay men suffered from various opportunistic diseases and some kinds of cancers. At the time the virus was not still names as AIDS or HIV, but the researchers and doctors were sure that all those people for suffering from the common syndrome.

Modes of transmission:
-Exchange of body fluids or unprotected sex.
-Blood transfusion.
-From mother to child at the time of pregnancy.
-Through open cuts.
-Unhygienic syringes
-through sharing of same blades while shaving
-tatoo piercing machines
-wanted injecting of the blood (some people who come to know that they are positive, behave pessimistically and inject their blood to other. this mainly occurs in cinema halls,shopping malls etc)