Human Immunodeficiency Virus (HIV)

Viruses are infectious agents that use a host cell to replicate. Human immunodeficiency virus (HIV) is a type of a virus known as a retrovirus, which is a class of viruses made up of RNA (ribonucleic acid) molecules. RNA is a variation of DNA, which comprises the molecular make-up of most cells' genetic material. Retroviruses use an enzyme called reverse transcriptase to make DNA copies of their RNA, which is then incorporated into the DNA of the host cell.

In many cases, there is a long time lag between initial infection with HIV and the development of symptoms. Based on several studies in the United States and other Western countries, the median time lag is 10 years, meaning that 50 percent of people infected with HIV develop symptoms before 10 years, and 50 percent develop symptoms after 10 years.

According to these same studies, approximately 10 percent of people infected with HIV develop symptoms within 2 or 3 years, and about 10 percent are asymptomatic (i.e., do not experience symptoms) after 12 years. A number of factors may account for this, including genetic differences among individuals, genetic differences in HIV strains, and the presence of other infections that affect the speed and effectiveness of HIV.

HIV-1 & HIV-2

There are two types of human immunodeficiency virus: HIV-1 and HIV-2. Unless specified otherwise, the term HIV generally refers to HIV-1.

HIV-1 and HIV-2 are associated with the same modes of transmission and both cause similar opportunistic infections and AIDS. Immunodeficiency may develop more slowly and may be milder in people infected with HIV-2. HIV-2 often is less infectious early in the course of infection and, compared to HIV-1, the duration of increased infectiousness is shorter. HIV-2 is most common in Africa; the United States has fewer reported cases of HIV-2 infection.

HIV-1 variations are grouped as "clades," which are geographically distributed. For example, B clade is restricted to North America, and E clade is found only in southeast Asia. There may be biological differences, in terms of virulence and ease of passage, among the clades as well.

A person can be infected with more than one clade of HIV-1. This is an important reason for practicing safe sex and not sharing needles, even if all parties are HIV positive. Re-infected with a virus from another clade can make the infection more difficult to treat and provide an opportunity for the two variants to combine and create yet a new variation of HIV. The more variants that develop, the more difficult it is to develop an HIV vaccine or find a cure for AIDS.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 14 Nov 2007

Last Modified: 12 Aug 2015