Basics
Human immunodeficiency virus (HIV) suppresses the immune system's ability to fight infection and disease. HIV was first identified in the early 1980s when doctors and public health officials began to notice clusters of previously unusual infections. The virus targets white blood cells in the immune system known as helper T cells (or CD4 cells), thus impairing the body's ability to fight infections. People infected with the HIV virus will gradually develop lower levels of CD4 cells and higher levels of HIV virus in their blood. CD4 cell levels are an important marker of HIV disease severity. The introduction and use of drug regimens to combat HIV infection has meant that many HIV-infected people with access to these drugs have a much-increased life expectancy than ever before.
HIV infection progresses through these stages: viral transmission, primary infection, seroconversion, clinical latent period with or without persistent generalized lymphadenopathy, early symptomatic disease, AIDS and advanced HIV infection.
- Viral transmission. This involves the introduction of the HIV virus from an already-infected person to someone who was not previously infected.
- Primary infection. This is the stage in which HIV first enters the body and begins reproducing itself. Primary infection may be accompanied by the sudden onset of flulike symptoms such as fever, joint pain, swollen lymph nodes, sore throat, mouth sores, nausea, diarrhea, and headache. Some people experience no symptoms at all. Symptoms generally occur two to four weeks after viral transmission occurs.
- Seroconversion. Seroconversion means that the virus will show up on blood tests. Most people exposed to HIV will seroconvert within four to 10 weeks, although it can take up to 6 months for some people.
- Asymptomatic disease. During this stage, which typically lasts from two to 10 years, HIV becomes established in the body, but does not cause symptoms. The number of CD4 cells in the blood begins a gradual decline, starting from around 1,000 cells per milliliter (mL) of blood to around 500 cells per mL.
- Symptomatic disease. When the CD4 count drops to between 200 and 500 cells per mL, patients may develop symptoms such as swollen glands, fatigue, unexplained weight loss, and fever. Infections that rarely occur in people with healthy immune systems also begin to appear.
- AIDS. AIDS isn't a single disease, but rather a group of symptoms or illnesses that occur together. AIDS has been defined by the U.S. Centers for Disease Control and Prevention (CDC) as occurring in a person who:
- Has a laboratory-documented HIV infection
- Has a CD4 count less than 200 cells per mL of blood
- Has had one or more infections or types of cancer that do not occur regularly in the general population.These infections include Candida (a yeast infection) of the esophagus or lungs, disseminated tuberculosis, PCP pneumonia, several bouts of bacterial pneumonia, and extrapulmonary coccidiomycosis and histoplasmosis. The types of cancer include invasive cervical cancer, Kaposi's sarcoma, and certain types of lymphoma. People without HIV develop cervical cancer and lymphoma, but anyone known to be infected with HIV who then develops one of these conditions is considered to have AIDS.
- Advanced AIDS. Anyone who has AIDS with a CD4 count less than 50 cells per mL has advanced AIDS. Survival at this stage is generally only 12 -18 months in people not taking medications to treat AIDS.
While the majority of people who contract HIV will ultimately develop AIDS, the time between initial HIV infection and the development of AIDS varies widely. Historically, most HIV-positive people develop AIDS within 10 to 11 years after infection; however, some people have lived with HIV for 15 years or more without symptoms. There currently is no cure for AIDS, though scientists are trying to develop a vaccine to prevent its spread. Highly active antiretroviral therapy (HAART), available in the developed world since 1996, has greatly prolonged the life expectancy of people living with AIDS who have access to this treatment.
Causes
HIV is spread primarily via contact with bodily fluids such as blood, semen, and vaginal secretions. HIV can also be spread from an infected mother to her fetus and via transplantation of infected organs. HIV cannot survive outside of the body for very long, and can only be transmitted through contact in which bodily fluids are exchanged. The primary means of HIV sexual transmission is through vaginal or anal intercourse, but HIV can also be transmitted through oral sex. HIV does not appear to be transmitted through kissing.
In the early years of the epidemic before effective tests for the virus were available, many people contracted HIV through blood transfusions, or from using blood-clotting factors, such as Factor VIII, used in hemophilia. Today, the risk of getting HIV from the blood supply is very small, particularly in the developed world in which donated blood undergoes rigorous testing. Blood-borne transmission of HIV occurs most commonly among people who share intravenous needles when using illicit drugs. HIV can spread via organ transplant, but rigorous testing of organ donors and the organ supply has greatly reduced the likelihood of this occurrence.
HIV can be passed from a mother to her child during pregnancy or childbirth, and after birth through breast milk. Viral transmission to the child may occur in the uterus. The risk of transmission is enhanced if there is a prolonged period of time between the rupture of a mother's membranes and the time of delivery. In addition, breast milk contains relatively high levels of HIV, and transmission of the virus from mother to child through breast-feeding has been well-documented. This route of transmission is especially problematic in developing countries where HIV-infected mothers may not have access to affordable, sterile, nutritious infant formula.
There are no documented cases in which HIV was transmitted through ordinary social contact. HIV transmission is not known to have occurred through touching, kissing, hugging, shaking hands, sharing food, being bitten by an insect, sitting on an infected toilet seat, or working or playing with an infected person.
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