One way HIV gets into your body is when you have unprotected sex with a person who already has the HIV virus. If you have the HIV virus in your body, it is called being HIV-positive. When people who are HIV-positive cannot fight viruses or infections anymore, they start to get sick more often. When this happens, we say that they have AIDS.
It can take many years from the time someone becomes HIV-positive to the time they have AIDS. But it is different for everyone. If an HIV-positive person takes their ARVs every day, and lives a healthy lifestyle, it is unlikely that their HIV will progress to AIDS.