The human immunodeficiency virus (HIV) is the virus that potentially causes AIDS (acquired immune deficiency syndrome). HIV primarily attacks the immune defense system, making the patient extremely vulnerable to opportunistic infections, which are infections that occur in people who have weakened immune systems.
HIV primarily infects and destroys immune cells with the CD4 receptor protein on their cell surfaces (also called CD4-positive or CD4+ T-cells). Healthy individuals have a CD4 cell count between 600 and 1,200 cells per microliter of blood. HIV patients have less than 600 CD4 cells per microliter of blood.
Patients progress to AIDS when/if their CD4 cell counts drop to lower than 200 cells per microliter (one-one millionth of a liter) of blood. This may happen if a person does not receive adequate treatment or if he/she develops a serious infection or illness. Individuals with a CD4 cell counts lower than 200 have the greatest risk of developing potentially fatal opportunistic infections, such as Mycobacterium avium complex (MAC) infections, because their immune systems are very weak.
HIV is transmitted from person to person via bodily fluids, including blood, semen, vaginal secretions, and breast milk. Therefore, it can be transmitted through sexual contact with an infected person, by sharing needles/syringes with someone who is infected, through breastfeeding, during vaginal birth, or, less commonly, through transfusions with infected blood. However, in countries where blood is screened for HIV antibodies, HIV infection is rarely transmitted through blood transfusions. Very low amounts of HIV have been found in saliva and tears in some AIDS patients. However, contact with saliva, tears, or sweat has not been shown to result in HIV transmission.
The most common type of HIV worldwide is called HIV-1. It is easily transmitted and is the cause of most HIV/AIDS infections around the world. HIV-1 has several subtypes (A through H and O), which are more common in certain parts of the world but produce AIDS similarly. The second type, called HIV-2, is much less common and less virulent or infectious.
Since 1981, when the first case of AIDS was reported in the United States, the disease has become a global pandemic, causing an estimated 65 million infections and 25 million deaths worldwide.
During the mid-to-late 1990s, advances in HIV treatment slowed the progression of HIV infection to AIDS. This consequently led to decreases in AIDS-related deaths. According to the U.S. Centers for Disease Control and Prevention (CDC), the number of AIDS-related deaths continues to decline, with an eight percent decrease from 2000 through 2004. However, the number of AIDS diagnoses increased eight percent during that period as well.
From 2000 to 2004, the estimated number of people living with AIDS increased from 320,177 to 415,193, according to the CDC. This 30% increase can partially be attributed to advanced treatments that have helped HIV/AIDS patients to live longer lives, as well as increased access to testing and information about the disease.
According to the CDC, an estimated 2.8 million patients died from AIDS, 4.1 million people became infected with HIV, and 38.6 million were living with HIV worldwide in 2005. According to the Joint United Nations Programme on HIV/AIDS (UNAIDS) and World Health Organization (WHO) 2006 AIDS Epidemic Update, an estimated 39.5 million people are currently living with HIV worldwide. It is also estimated that 4.3 million people became newly infected in 2006, with 2.8 million (65%) of these cases occurring in Sub-Saharan Africa. In 2006, 2.9 million people died from AIDS-related illnesses.
Certain geographic regions, such as Sub-Saharan Africa and the Caribbean, have much higher rates of infection than others. Certain populations, such as Sub-Saharan women, men who have sex with men (MSM), prostitutes, and injection-drug users, are also at increased risk for HIV infection.
Currently, there is no cure for HIV/AIDS. However, treatment with anti-HIV drugs, called antiretrovirals, may suppress the virus, which subsequently helps boost the immune system. Although these drugs may help patients live longer lives, they do not reduce the risk of transmitting the disease to someone else.

Related Terms

Acquired immune deficiency syndrome, acquired immunodeficiency syndrome, AIDS, antiretroviral, antiretroviral therapy, ART, AZT, CD4 T-cells, CD4+ T cells, CD8 T-cells, CD8+ T cells, consumer-controlled test kits, Durban Declaration, fusion inhibitor, HAART, highly active antiretroviral therapy, HIV-1, HIV-2, hivid, home testing kit, human immunodeficiency virus, IDU, immune, immune defense system, immune response, immunocompromised, immunodeficiency, injection drug use, Kaposi's sarcoma, lymphocytes, MAC infections, Mycobacterium avium complex infections, non-nucleoside reverse transcriptase inhibitor, nucleoside reverse transcriptase inhibitor, opportunistic infection, PCP, Pneumocystis carinii pneumonia, Pneumocystis jiroveci pneumonia, protease inhibitor, rapid test, retrovirus, sexually transmitted diseases, sexually transmitted infections, SIDA, STD, STI, viral infection, virus, weakened immune system, white blood cells.


General: HIV is transmitted from person to person via bodily fluids including blood, semen, vaginal secretions, and breast milk. Individuals who have other sexually transmitted diseases are more susceptible to the virus. This is because some STDs, such as syphilis, cause breaks in the skin or sores that make it easy for HIV to enter the body.
HIV is particularly difficult to treat because it reproduces very quickly and has a high mutation rate. When HIV reproduces, different strains (types) of the virus emerge. Mutations (changes in genetic information) occur almost every time a new copy of the virus is produced. Therefore, many types of HIV can be produced in a single person in one day. For this reason, HIV patients receive different combinations of antiretrovirals to suppress the virus.
HIV primary targets immune cells called CD4 T-cells. The CD4 T-cells are white blood cells that help coordinate the immune system's response to infection and disease. These cells have a molecule called CD4 on their surfaces, which allows the cells to detect foreign substances, including viruses that enter the body. This process then triggers the immune system to destroy the foreign substance. When HIV enters the body, the virus recognizes this protein, binds to the receptors on the CD4 cell wall, and enters the cell. Once inside the cell, HIV replicates and eventually kills the cell.
Primary, or acute, infection: Patients can transmit the virus to others during all stages of infection. The first stage of HIV, known as the primary or acute infection, is the most infectious stage of the disease, and it typically lasts several weeks. During this phase, the virus replicates rapidly, which leads to an abundance of the virus in the bloodstream and a drastic decline in the number of CD4 T-cells. The CD8 T-cells, which kill abnormal or infected body cells, are then activated to destroy HIV-infected body cells and antibodies are produced.
Clinical latency: The next stage, called clinical latency, may last anywhere from two weeks to 20 years. During this phase, HIV is not considered dormant. Instead, it is active in the lymph nodes where large amounts of the virus become trapped. The surrounding tissues, which contain high levels of CD4 T-cells, may also become infected. The virus accumulates in infected cells and in the blood as free virus. Patients generally do not experience symptoms during this stage until the CD4 cell count drops to 600 microliters of blood or lower.
Symptomatic stage: As the virus continues to weaken the immune system, patients eventually become more susceptible to infections. The next stage is the symptomatic stage, in which the person experiences symptoms associated with a weakened immune system.
AIDS: The term AIDS (acquired immune deficiency syndrome) refers to the most advanced stage of HIV infection. This stage happens when HIV multiplies rapidly and severely affects the immune system. Individuals have AIDS when they have fewer than 200 CD4 T-cells per microliter of blood. This low CD4 T-cell count makes them extremely vulnerable to potentially fatal opportunistic infections such as pneumonia or tuberculosis.
Several infections, including Pneumocystis jiroveci pneumonia (formerly called Pneumocystis carinii pneumonia, PCP) and Kaposi's sarcoma (KS), are considered AIDS-defining illnesses. This means that once a patient develops one of these infections, their condition has progressed to AIDS.


General: Race and ethnicity by themselves do not increase or decrease a person's risk of acquiring HIV infection. However, certain people are more likely to face challenges associated with the risk for HIV infection, such as lack of awareness of HIV status, substance abuse, or socioeconomic issues.
African Americans: In 2004, African Americans accounted for 20,965 (49%) of the 42,514 estimated AIDS cases (not HIV) diagnosed in the United States. The rate of AIDS diagnoses among African American adults and adolescents was 10 times higher than the rate of Caucasians and almost three times higher than the rate of Latinos. In addition, 23 times more African American women were diagnosed with AIDS than Caucasian women. Eight times more African American men were diagnosed with AIDS than Caucasian men, according to the CDC.
Latinos: In 2004, Latinos accounted for 20% (8,672) of the 42,514 new diagnoses in the United States, according to the CDC. The top causes of infection in most Latino men were infections after HIV exposure through sexual contact with other men, injection drug use, and heterosexual contact. The top causes of infection in Latino women were HIV exposure through heterosexual contact and injection drug use. Latinos were shown to get tested for HIV more often than any other race or ethnicity except African Americans. According to the CDC, in 2004, about 50% of Hispanics between the ages of 15 and 44 were tested, and 18% had been tested during the past year.
Women: After the initial outbreak of HIV, few women were diagnosed with the virus. Today, women account for more than 25% of all new HIV/AIDS diagnoses in the United States. In 2004, an estimated 93,566 women were living with AIDS, making up 23% of the estimated 415,193 people living with AIDS in the United States.
Youth (13-24 years old): In the United States, it is estimated that 50% of the 40,000 new HIV infections each year occur in people younger than age 25 and 25% of infections occur in people younger than 21. HIV/AIDS ranks as the sixth-leading cause of death among individuals ages 15-24 in the United States, with the number of AIDS cases reported each year in that age group increasing by 417% from 1981 through 1994.
Injection-drug users: Since the AIDS epidemic began, injection drug use (IDU) with illegal drugs has directly and indirectly (drug use clouds judgment, leading people to engage in high-risk behaviors) accounted for more than one-third of AIDS cases in the United States. In the year 2000, out of the 42,156 new cases of AIDS reported, 11,635 were linked to IDU.
Injection drug use is more common among racial and ethnic minorities in the United States, which makes them more likely to acquire HIV through IDU. In 2000, IDU accounted for 26% of all AIDS cases among African American adults and 31% among Hispanic adults and adolescents, compared to 19% of all cases among Caucasian adults and adolescents.
Homosexuals: Men who have sex with men (MSM) accounted for 70% of all estimated HIV infections among male adults and adolescents in 2004 in the United States, according to the CDC. Although the number of HIV diagnoses for MSM decreased during the 1980s and 1990s, the number increased by eight percent from 2003 through 2004. According to the CDC, it is unknown whether this increase is because more people are getting tested for HIV or because more patients are becoming infected with HIV.
Healthcare workers: Although healthcare workers are exposed to the virus at work, it is unlikely that they will acquire the virus from a patient, especially if they follow universal precautions, which should be taken with all patients.
For healthcare workers, HIV transmission is most likely to occur through accidental injuries from needles or other sharp medical instruments that may be contaminated with the virus. However, this risk is small. Researchers estimate that about 0.3-1% of healthcare workers exposed to the virus by an accidental needle stick or puncture develop HIV.
Since December 2001, there have been 57 documented reports of healthcare workers acquiring HIV from a patient. To prevent transmission of HIV to healthcare personnel in the workplace, the U.S. Centers for Disease Control and Prevention (CDC) offers precautionary guidelines.


Bodily fluids: HIV is transmitted from person to person via bodily fluids because the virus is present in varying concentrations in the blood, semen, vaginal fluid, and breast milk. It can be spread by sexual contact with an infected person, by sharing needles/syringes with someone who is infected, or, less commonly, through transfusions with infected blood. HIV infection through blood transfusion is extremely rare in countries where blood is screened for HIV antibodies.
Environment: HIV does not survive well in the environment outside of the body. According to studies performed by the U.S. Centers for Disease Control and Prevention (CDC), drying HIV reduces the amount of viral particles by 90-99% within several hours. The virus also cannot reproduce when it is outside of the body. Therefore, it is highly unlikely that the disease can be transmitted through contact with the environment, such as public toilet seats.
Kissing: Casual contact through closed-mouth or "social" kissing does not put an individual at risk for HIV. However, there is the potential for blood contact with open-mouth kissing. The risk for acquiring the virus from open-mouth kissing is low, and the CDC has only investigated one case in which HIV transmission may have been caused by blood contact during open-mouth kissing. The CDC recommends that individuals avoid open-mouth kissing with an infected person.
Biting: There have been medical reports that found that HIV transmission resulted after a human bite. Severe trauma and extensive tissue tearing were reported in each of these cases. However, biting is not a common way of transmitting the disease.
Saliva, tears, and sweat: Very low amounts of HIV have been found in the saliva and tears of some AIDS patients. However, a small amount of HIV in body fluid does not necessarily mean that the fluid can transmit the virus. Contact with saliva, tears, or sweat has not been shown to result in transmission of HIV.
Insects: According to numerous studies, there is no evidence to suggest that HIV has been transmitted through insects, even in areas such as Africa, that have high numbers of AIDS patients and mosquito populations. HIV can only live for a short time inside an insect and does not reproduce inside insects.
Effectiveness of condoms: If a condom is used properly during sexual intercourse, an individual may reduce the risk of acquiring or transmitting STDs, including HIV. Several studies show that using condoms correctly and consistently may reduce the breakage rates of latex condoms to less than two percent.
There are many types and brands of condoms, but only latex or polyurethane condoms have been shown to effectively prevent HIV transmission when used appropriately. According to the CDC, natural membrane condoms, such as those made with lambskin, have natural pores that can possibly transmit diseases. Therefore, lambskin condoms are not considered to be effective in preventing HIV transmission.
Recent evidence has suggested that condom use by high-risk populations increases, rather than decreases the infection rate. According to the latest studies, condom promotion is only effective in lowering the rate of AIDS in concentrated, high-risk groups; condoms have never been shown to reduce HIV infection rates and AIDS deaths in general-population epidemics like those in sub-Saharan Africa. In on study, researchers asserted that of the three interventions scientifically shown to prevent AIDS - abstinence, being faithful, and using condoms - they argue that the use of condoms clearly comes last and should be promoted as a first-line defense only to those in extremely high-risk groups, such as commercial sex workers.
One prospective study showed that for 'receptive' men during anal sex, it made little or no difference whether their partners used a condom or not. Researchers suggested that condoms are less effective in anal sex than in vaginal sex.