A decrease in the body’s sensitivity to (and effectiveness of) a drug. Resistance is the result of mutation (change in the genetic code of the virus). Resistance to HIV antiretroviral drugs usually arises due to lack of proper adherence (i.e. skipping and missing doses). If resistance emerges the drug cannot as effectively target HIV enzymes (proteins that regulate the replication of HIV) that they are designed to. A mutation that alters the structure of a viral enzyme will decrease the virus’s sensitivity to the drug. When this occurs the drug will be less able to continue to supress the replication of the mutated virus, and the virus will still be able to replicate, leading to treatment failure and increased HIV viral load. Drug resistance can also develop in other viruses, bacteria, fungi and protozoa.