What is the difference between HIV and AIDS?
In simple terms - you can't 'catch' AIDS. You can, however, catch HIV. Being infected with HIV does not mean that one has AIDS, but if left undiagnosed and/or untreated, HIV infection damages the immune system and can progress to AIDS.
AIDS results from the desruction of the immune system by HIV. The immune system's function is to fight off infections and other diseases. If your immune system is damaged or not working well, you are at risk of life-threatening infections and cancers. HIV attacks and destroys the disease fighting cells of the immune system. The body is left with a weakened defense against disease.