Once a person gets HIV, the virus begins to multiply in the body. As HIV multiplies, it sometimes changes form (mutates). Some HIV mutations that develop while a person is taking HIV medicines can lead to drug-resistant HIV. Once drug resistance develops, HIV medicines that previously controlled the person’s HIV are no longer effective. In other words, HIV medicines can’t prevent drug-resistant HIV from multiplying. Drug resistance can cause HIV treatment to fail. Drug-resistant HIV can spread from person to person (called transmitted resistance). People with transmitted resistance have HIV that is resistant to one or more HIV medicines even before they start taking HIV medicines.