When HIV gets resistant to one medicine, this is changed to another medicine. So the "AIDS cocktail" that people with AIDS take changes over time. But after a long time, the HIV learns to be resistant to "many" drugs. This is called "multi-drug-resistant" (acronym MDR) HIV. After the HIV in a person has MDR-HIV there may be no more medicines to treat them. So scientists keep trying to find new medicines to fight HIV.