An alteration (change) in the genetic material (DNA or RNA). Most mutations are natural processes that occur during cell replication. The alteration in the genetic material is permanent and is carried during replication to new generations. Mutations also arise in viruses, bacteria, fungi, protozoa as well as human cells. HIV is highly susceptible to mutation, and mutations are a major cause of the development of HIV drug resistance. HIV is less able to mutate when combination HIV antiretroviral treatments are used, and adherence is high (taking the medications daily as prescribed, without missing doses).