However, we still do not know when this happened exactly. The year is unknown, but we can guess that this happened several decades ago.
The first case of HIV infection in the United States was reported in the beginning of 1980s, when five homosexual males were hospitalized for severe pneumonia. These people died and doctors weren't able to tell what it was about. Researches started and medical analysis showed that these deaths were caused by something more serious than common pneumonia.
In 1986, the term Human Immunodeficiency Virus was officially accepted in medical science. The panic this disease caused was enormous and people knew very little about it, which only contributed to further spreading. The cure and prevention were also little known about.
Today, we know more about HIV treatment. However, its origin is less important. The most important thing is to find the cure.