When HIV multiplies, many of the new copies have mutations: they are slightly different from the original virus. Some mutant viruses keep multiplying even when you are taking ARV drugs. When this happens, the virus can develop resistance to the drug and ART may stop working.

• If only one or two ARV drugs are used, it is easy for the virus to develop resistance. For this reason, using just one or two  drugs is not recommended. But if two or three drugs are used, a successful mutant would have to “get around” all of the drugs at the same time. Using combination therapy means that it takes much longer for resistance to develop.