HIV-AIDS
HIV/AIDS are two things, HIV and AIDS. HIV is human immunodeficiency virus and AIDS is acquired immune destroying syndrome. HIV is a sexually transmitted infection that causes AIDS. AIDS can kill you, so this STI is deadly.
You can get HIV from used needles from drug use, from blood transfusions, from vaginal and anal sex, and pregnancy, birth and breast milk.
The HIV is a tiny virus that infects the white blood cells of the body. The white blood cells are in charge of your immune system. If they're wiped out, your body cannot fight off virus infections. There is no vaccine or cure for HIV. But if you get modern medical treatment you will probably not die of AIDS.
HIV can only be prevented, not cured. To prevent it practice safer sex (use condoms?) and don't be promiscuous?.
This disease would mostly or entirely disappear from the earth if we just stopped doing things wrong.
In the country of South Africa there is a taboo on mentioning that HIV leads to AIDS. I don't know how to help with that, because it definitely does (if untreated).