HIV Infection Facts, History and Causes

Human immunodeficiency virus (HIV) facts

  • HIV is the virus that causes HIV infection and the acquired immunodeficiency syndrome (AIDS).
  • Anal or vaginal sexual intercourse and illicit injectable drug use commonly transmit HIV. Infected mothers may also transmit HIV to their child during pregnancy, delivery, or breastfeeding. Less common routes of transmission include needle-stick injuries or exposure to contaminated blood.
  • The blood supply in the United States is tested for HIV before use, and statistics show the risk of acquiring HIV infection from a transfusion is less than one in 1.5 million.
  • HIV attacks the immune system, especially cells known as CD-4 lymphocytes. Serious impairment of the CD-4 lymphocytes makes people susceptible to specific infections and cancers.
  • Untreated HIV progresses from an acute retroviral syndrome with high levels of HIV in the blood to a period of silent infection from months to several years and finally to symptoms of immune dysfunction (AIDS), progressive weight loss, and ultimately death.
  • Health care professionals diagnose HIV with tests that detect antibodies against the virus and/or antigen (particles of the virus) or directly measure the quantity of virus in the blood.
  • Treatment with highly active antiretroviral therapy (HAART or ART) dramatically increases life expectancy although it does not cure HIV infection.

What is the human immunodeficiency virus?

There are two types of HIV: HIV-1 and HIV-2. HIV-1 is the primary cause of HIV infection and AIDS in the world. HIV-2 is limited to parts of West Africa and is less easily transmitted.

The human immunodeficiency virus is the cause of HIV infection and the acquired immunodeficiency syndrome (AIDS). HIV belongs to a family of viruses known as retroviruses. Once someone acquires the virus, it attaches to and enters human cells, especially immune defense cells known as CD4 T-cells, macrophages, and dendritic cells. The virus contains RNA, which it transcribes into DNA using an enzyme called reverse transcriptase. The resulting DNA permanently integrates into the human genome in the cell. By inserting its own genetic instructions into a cell's DNA, the virus fools the human genome into making more copies of the virus. Infected cells produce copies of HIV virus until they burst. Millions of cells release trillions of new HIV viruses every day, and these infect more and more cells.

During times of peak viral reproduction, even 1 milliliter of blood can contain more than 1 million copies of the virus. Each HIV virus produced has slightly different proteins on its surface. Because of continual high volume and rapid changes in surface proteins, the body cannot detect and kill these invaders before many new cells are hijacked and killed. In addition, the viruses may be produced with mutations that make them resistant to different treatments. Furthermore, there are areas of the body, such as sex organs and brain, which are more difficult to reach with medications and can serve as persistent reservoirs of the virus. So, while HIV production can be controlled by treatment, there is currently no way to stop all virus production until the infected cells die and are completely replaced with healthy ones.


What is HIV? See Answer

What is the history of HIV?

Although HIV infection is only a recent discovery, scientists have shown that HIV-1 may have spread to humans over 100 years ago. The likely source was primate-to-human transmission through bites or blood exposure during hunting and eating the meat, with chimpanzees being the most likely candidate for HIV-1 transmission to humans.

Isolated human cases existed long before HIV infection came to the attention of physicians. It is not entirely clear what caused HIV to begin to spread more widely in the mid-20th century. HIV has been identified in tissue preserved from patients who died as early as 1959. In the late 1960s or early 1970s, it is thought that HIV infection spread to Haiti and then to the United States. Recognized in the gay male community initially, the virus began to spread among people with multiple sexual partners. Over time, often a decade after infection, people with HIV infection began to get unusual infections, develop AIDS, and die.

In 1981, the Centers for Disease Control and Prevention first took note of a cluster of pneumonia causes by Pneumocystis jirovecii (formerly known as Pneumocystis carinii), which was a highly unusual pathogen to find in young men. Affected individuals were noted to have depleted counts of a specific type of immune cell, known as the CD-4 T cell. Subsequent investigation revealed that gay men with multiple partners were most likely to have disease, suggesting that sexual activity was a key factor in transmission. Soon, scientists discovered the disease in people injecting illicit drugs, in heterosexuals with multiple sexual partners, in people who received blood products, and in babies born to infected mothers. A global effort was undertaken to identify the virus, leading to the description of HIV as the likely cause of AIDS by Dr. Luc Montagnier in France and by Dr. Robert Gallo in the United States. A test for HIV was soon developed and used to diagnose individuals and to ensure the safety of the blood supply.

While some still associate HIV with the gay male community, HIV does not discriminate by class, gender, ethnicity, or sexual orientation. It circulates among injection drug users (especially with resurgence of heroin use) of all backgrounds and incomes, among people of any gender-orientation who practice high-risk behaviors for acquiring HIV, and it is important to prevention efforts to remember this.

What causes an HIV infection?

Almost 37 million people are HIV-infected worldwide. Although this number has dropped in the last decade, Sub-Saharan Africa still bears the highest burden with over 2 million new HIV infections as of 2014. Undiagnosed HIV infection is responsible for continued transmission, even up to one-third of transmissions in the U.S. of 1.2 million estimated cases in individuals over 13 years of age as of December 2012. Using scientific models, the CDC estimated that 14% of them probably did not know they were infected. The South bears the highest burden in terms of numbers of HIV+ cases per 100,000 people. In 2015, African-Americans made up 12% of the U.S. population but had nearly half of new HIV diagnoses. Though Hispanic populations made up 18% of the population in 2015, they experienced almost one-quarter of new HIV diagnoses.

Picture of new HIV diagnoses by race/ethnicity in 2015

In 2015, MSM (men who have sex with men) accounted for 67% of all new HIV diagnoses; however, 24% of all new cases affected people via heterosexual sex, including 86% of newly infected women. Of greatest concern, is the rise in proportion of young people acquiring HIV. Young gay and bisexual men accounted for 84% of all new HIV diagnoses in people aged 13-24 in 2015; young African-Americans are the most heavily impacted in this group.

Picture of new HIV diagnoses in the United States for the most-affected subpopulations, 2015
Picture of new HIV diagnoses in the United States for the most-affected subpopulations, 2015; Source: CDC

Only 6% (2,692) of new diagnoses were attributed to injection drug use (IVU), including 13% of women with new HIV infections.

What are risk factors for an HIV infection?

The most common modes of transmission in the United States are via

  • anal sexual intercourse,
  • vaginal sexual intercourse,  and
  • use of injected drugs where needles/syringes/materials are shared with others.

Thus, the main risk factors for HIV infection are having non-monogamous sexual intercourse, having unprotected sex (not using condoms), or the use of injectable illicit drugs. Anal sex is the riskiest form for getting or transmitting HIV for both men and women, especially for the partner receiving the penis. Sex without a condom or other barrier to genital fluids is also a risk factor for several other sexually transmitted infections (STIs), which themselves can increase the risk of HIV infection due to inflammation of tissues. There is an extremely low risk of getting or transmitting HIV through oral sex (mouth to genitals or anus), kissing, or touching, though other STIs are still possible. There is also a risk of transmission from an infected mother to an unborn child during pregnancy or delivery. All pregnant women should be tested for HIV infection. While 8,000 HIV-infected women give birth yearly, appropriate use of anti-HIV medications has dropped the risk of HIV infection in newborns to 1% or less. Women with HIV-infected male partners may take anti-HIV medications while trying to get pregnant as well. There is also some risk in the perinatal period because breast milk may harbor HIV, and infected mothers in the United States are counseled not to breastfeed and not to pre-chew their baby's food. In developing countries, alternatives to breast milk may be lacking and recommendations may differ. Currently, there are approximately 200 new HIV cases per year in the U.S. in children younger than 13 years of age, and most (75%) were infected after birth.

Other modes of transmission include accidental needle stick or sharps injuries in health-care workers and splashes of contaminated material onto mucous membranes or nonintact skin. Risk factors include unsafe practices for handling used needles or sharps, inappropriate disposal of needles/sharps, and inadequate use of personal protective measures such as gloves, masks, and eye protection when there is a chance of splashing.

The blood supply in the United States is tested for HIV before use and the risk of acquiring HIV infection from a transfusion is less than one in 1.5 million.

Casual contact (shaking hands, sneezing, coughing, sharing drinking glasses, sharing a toilet, exposure to saliva from social kissing, etc.) does not pose a threat for HIV infection.

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United States. Department of Health and Human Services. "Guidelines for the use of antiretroviral agents in HIV-1-infected adults and adolescents." July 14, 2016. <>.