The Centers for Disease Control and Prevention (CDC) estimates more than 1.1 million Americans were living with HIV/AIDS as of 2007. Two pathogens cause the illness known as acquired immunodeficiency syndrome (AIDS): HIV-1 and HIV-2. HIV-1 is responsible for the overwhelming majority of HIV/AIDS worldwide. Although early diagnosis and treatment have markedly reduced the number of HIV-related deaths in the U.S., the disease remains a significant cause of chronic illness. The characteristics of the human immunodeficiency virus differentiate it from all other viruses that afflict mankind.
Capacity for Protracted Infection
HIV (the human immunodeficiency virus) belongs to the Retroviridae virus family in the subfamily Lentiviridae. The lentiviruses, including HIV, have the unusual characteristic of causing protracted, slowly progressive infections. A prolonged, largely asymptomatic incubation period of months to years occurs before the onset of the definitive, symptomatic illness. Current HIV antiviral therapy is aimed at keeping HIV infection from reaching the definitive illness stage, AIDS, which is characterized by multiple opportunistic infections, wasting and, ultimately, death.
High Degree of Genetic Variability
HIV exhibits an unusually high degree of genetic variability due to a rapid mutation rate. A mutation is a minor change in the genetic material of the virus. Although these changes are minor, they cause significant differences, especially on the viral surface. The existence of a number of HIV groups and subtypes demonstrates the genetic variability of HIV. There are three main HIV-1 groups: M (main or major), N (new) and O (outlier or other). The majority of HIV infections worldwide are caused by HIV-1 group M. The M group is further broken down into subtypes A through K. The predominant HIV-1 group M subtype in North America is B.
Presence of a Viral Envelope
HIV particles are comprised of a core and an envelope. The core contains the genetic material and enzymes of the virus. The envelope is the outer layer of the viral particle, which interacts with human cells, enabling the virus to gain entry and infect them. The HIV envelope is made of a fatty material with proteins named gp120 and gp41 projecting from its surface. The HIV surface protein gp120 is particularly important, as it is the protein that facilitates binding of the virus to its human target cells.
Capacity to Integrate Viral Genes into Human DNA
The genetic material of HIV is composed of ribonucleic acid, or RNA. Human genetic material is composed of deoxyribonucleic acid, or DNA. The fact that HIV genes are RNA and not DNA is important, because it necessitates the HIV RNA be translated into DNA to enable the virus to take over a human cell. The process of creating a DNA version of the HIV RNA is accomplished by an HIV enzyme called reverse transcriptase. The DNA copy of HIV integrates into the human cell's DNA, an unusual viral characteristic that contributes to the longevity of HIV infection. HIV's reverse transcriptase enzyme is a key target for HIV antiviral therapy. Nucleoside reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) are antiviral medicines commonly used to treat HIV. These drugs block the conversion of HIV RNA into DNA by inhibiting the HIV reverse transcriptase enzyme.
References
- "Principles and Practice of Infectious Diseases, Sixth Edition"; Gerald L. Mandell, M.D., et al; 2004
- Microbiology and Immunology Online, University of South Carolina School of Medicine: Human Immunodeficiency Virus and AIDS
- Merck Manual: Human Immunodeficiency Virus
- HIV Medicine: HIV Medicine 2007: Pathogenesis of HIV-1 Infection
- Centers for Disease Control and Prevention: HIV/AIDS Basic Statistics
