The level of HIV resistance to an antiretroviral medication can increase even though appropriate drug treatment is being administered. In the absence of antiretroviral treatment or immunological control, HIV replicates freely in the lymphatic tissue - producing as many as 10 million new virus particles a day. The virus incorporates occasional errors during this replication, resulting in progeny HIV whose genetic material is not identical to the original. HIV incorporates a single change in the genetic sequence almost every time a new copy is made, but not every mutation causes resistance. In some cases a single mutation can confer resistance to a drug, while in other cases a set of mutations is required before a drug becomes ineffective.
Antiretroviral drugs control the replication of most variants of the virus.
However, HIV virus that contains a mutation causing resistance to a drug one is taking will eventually multiply and become the most common form in one's body. If one stops taking antiretroviral medications, there is no selective drug pressure. However, in most cases, drug-resistant mutations won't disappear entirely. They can reappear quickly if one takes the same drugs again. While just one mutation can cause resistance to some drugs, in most cases HIV has to go through a series of mutations to develop resistance to other drugs, including most protease inhibitors.