HIV/AIDS is caused by the human immunodeficiency virus (HIV) which weakens the immune system and allows opportunistic infections. It was first observed in the 1980s and has since caused over 25 million deaths globally, with about 40 million people currently living with HIV. The virus is transmitted through sexual contact, sharing needles, or from mother to child during birth or breastfeeding. While treatment can suppress HIV and reduce transmission, prevention through testing and safe practices is the best way to end the pandemic.