HIV drug-resistance mutations can occur both before and during HIV treatment. Here's a look at how this happens:
Transmission of drug-resistant HIV. Many HIV-positive people now take HIV drugs. If someone has developed resistance to one or more of these HIV drugs and has unprotected sex or shares needles with someone who is not infected with the virus, it is possible that they can infect their partner with a drug-resistant variant—a strain of HIV containing mutations that can cause resistance.
In the United States and other countries where HIV treatment is widely used, between 5 percent and 20 percent of new HIV cases involve strains of the virus that are resistant to at least one HIV medication.
During treatment. Even if someone is infected with HIV that doesn't contain drug-resistance mutations ("wild-type" virus), genetic changes still occur over time, even before treatment is started. This ends up creating a large mixture of virus in the body. Some of these variants contain the necessary mutations that can partially, or fully, resist an antiretroviral drug—which explains why one-drug treatment (monotherapy) should never be used to treat HIV.
Soon after combination HIV drug treatment is started, the amount of wild-type virus is dramatically reduced. However, if the amount of virus isn't pushed down and kept at very low levels, HIV variants can continue replicating, acquiring additional mutations. And once the virus has accumulated enough mutations, a high level of resistance to the drugs being used can occur, causing viral load to increase and CD4 cells to drop.
A major concern with these mutations is that they can result in cross-resistance. This means that HIV resistance to one drug can automatically become resistant to other drugs in the same class. For example, if you're on a drug regimen that contains the NNRTI Sustiva and your virus becomes resistant to it, chances are that your virus is also resistant to the NNRTIs Viramune and Rescriptor, even though you haven't taken these drugs.
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