There are two types human immunodeficiency virus (HIV):

HIV-1, which causes almost all the cases of AIDS worldwide.
HIV-2, which causes an AIDS-like illness. HIV-2 infection is uncommon in the United States.

How the disease is spread
HIV is spread when blood, semen, or vaginal fluids from an infected person enter another person's body, usually through:

Sexual contact. The virus may enter the body through a tear in the lining of the rectum, vagina, urethra, or mouth. Worldwide, between 75% and 80% of all cases of HIV are transmitted by sexual contact.

Infected blood. HIV can be spread when a person:
Shares needles, syringes, cookers, cotton, spoons, or eyedroppers used for injecting illegal drugs or steroids.
Is accidentally stuck with a needle or other sharp item contaminated with HIV.

It is now extremely rare in the United States for HIV to be transmitted by blood transfusions. Blood donors are screened for risk factors. All donated blood is screened for HIV antibodies, and most blood products are heat-treated to kill any HIV virus that may be present.

Health care workers are no longer considered to be at high risk of exposure to HIV. Policies are in place in health facilities that require protection from accidental exposure. Sharp objects must be properly disposed of, along with wearing protective gloves, gowns, and eye and face protection (universal precautions). These measures have proven effective in protecting health care workers from HIV.

Spread of HIV to babies
A woman who is infected with HIV can spread the virus to her baby during pregnancy, delivery, or breast-feeding.

Most children younger than 13 who have HIV were infected with the virus by their mothers.

The risk of a woman spreading HIV to her baby can be greatly reduced if she takes the medication zidovudine (ZDV, formerly AZT) during pregnancy and if she does not breast-feed her baby. The baby should also receive ZDV after it is born.

Ways HIV cannot be spread
HIV does not survive well outside the body. Therefore, HIV cannot be spread through casual contact such as sharing drinking glasses or by casual kissing with an infected person. HIV is not transmitted through contact with saliva, sweat, tears, urine, or feces, or through insect bites.

Contagious and incubation period
Even after being infected with HIV, there is a period of time during which it does not show up in blood tests-the body has not yet produced enough HIV antibodies to be detected. This period of time is called the seroconversion period. However, an infected person can still pass HIV to another person during this time. The seroconversion period can be as short as 2 weeks or as long as 6 months.

Once infected with HIV, blood, semen, or vaginal fluids are always infectious, even if the person receives treatment for the HIV infection.

Stages of HIV
Most people go through the following stages after being infected with HIV:

Acute retroviral syndrome, which is a flulike illness. This usually occurs 3 to 6 weeks after exposure to HIV but can occur within a few days of exposure.

HIV without symptoms (asymptomatic). It may take years for HIV symptoms to develop. However, even though no symptoms are present, the virus is multiplying (or making copies of itself) in the body during this time. HIV multiplies so quickly that the immune system cannot destroy the virus. After years of fighting HIV, the immune system starts to weaken.

HIV with symptoms (symptomatic). Once the immune system starts to weaken, other infections can easily enter the body, such as opportunistic infections (for example, pneumonia or tuberculosis).

AIDS, which occurs during the last stage of infection with HIV. If HIV goes untreated, AIDS develops in most people within 12 to 13 years after the initial infection. With treatment for HIV, the progression to AIDS may be delayed or prevented.

A small number of people who are infected with HIV are rapid progressors. They develop AIDS within about 3 years if they do not receive treatment. It is not known why the infection progresses faster in these people. They may have already had another problem with their immune system.

Nonprogressors and HIV-resistant
A few people with HIV are described as nonprogressors. Most nonprogressors who have HIV that does not progress to more severe symptoms or disease:

Have lived with the infection for 10 to 15 years and remained healthy.
Do not have declining CD4+ cell counts.
Have a very low level of HIV in their blood.

A small number of people never become infected with HIV despite years of exposure to the virus; for example, they may have repeated, unprotected sex with an infected person. These people are said to be HIV-resistant.

Studies are under way to determine why some people either do not become infected with HIV or, if they do, why they do not develop symptoms or lose CD4+ cells. Research has shown that:

Some people's CD4+ cells are relatively resistant to HIV. If HIV cannot attach itself to CD4+ cells, it cannot destroy them.
Some people's immune systems may be better able to destroy the virus.
Some strains of HIV may not be as harmful.