In an attempt to survive and fight back, the virus will continue using the mutation against the drug and drug resistance occurs. In HIV, if the virus has normal genetic information, it will most likely be sensitive to antiviral medications. If the virus has mutated genetic information, it may be resistant to an antiviral medication. In HIV, phenotype normally refers to drug resistance or susceptibility. In other words, the virus is no longer vulnerable to a particular antiviral medication.
Viral resistance is thought to be the result of genetic mutations.
In the case of HIV, the current resistance tests need the patient to have a particular viral load in order to work. They also need a resistant minor species to make up a particular amount of the virus in order to find it. A No-Nonsense Guide to HIV Drug Resistance Testing . University of California, San Diego. Geno- and Phenotypic Resistance Tests.