HIV (human immunodeficiency virus) is the virus that causes AIDS. This virus is passed from one person to another through blood-to-blood and sexual contact. In addition, infected pregnant women can pass HIV to their babies during pregnancy or delivery, as well as through breast-feeding. People with HIV have what is called HIV infection. Most of these people will develop AIDS as a result of their HIV infection.

We do not know. Scientists have different theories about the origin of HIV, but none have been proven. The earliest known case of HIV was from a blood sample collected in 1959 from a man in Kinshasha, Democratic Republic of Congo. (How he became infected is not known.) Genetic analysis of this blood sample suggests that HIV-1 may have stemmed from a single virus in the late 1940s or early 1950s. We do know that the virus has existed in the United States since at least the mid to late 1970s.

HIV transmission can occur when blood, semen (including pre-seminal fluid, or "pre-cum"), vaginal fluid, or breast milk from an infected person enters the body of an uninfected person. HIV can enter the body through a vein (e.g., injection drug use), the anus or rectum, the vagina, the penis, the mouth, other mucous membranes (e.g., eyes or inside of the nose), or cuts and sores. Intact, healthy skin is an excellent barrier against HIV and other viruses and bacteria.

HIV is not casually transmitted, so kissing on the cheek is very safe. Even if the other person has the virus, your unbroken skin is a good barrier. No one has become infected from such ordinary social contact as dry kisses, hugs, and handshakes.

No. From the start of the HIV epidemic there has been concern about HIV transmission of the virus by biting and bloodsucking insects, such as mosquitoes. However, studies conducted by the CDC and elsewhere have shown no evidence of HIV transmission through mosquitoes or any other insects -- even in areas where there are many cases of AIDS and large populations of mosquitoes.

Since 1992, scientists have estimated that about half the people with HIV develop AIDS within 10 years after becoming infected. This time varies greatly from person to person and can depend on many factors, including a person's health status and their health-related behaviors. Today there are medical treatments that can slow down the rate at which HIV weakens the immune system.

Open-mouth kissing is considered a very low-risk activity for the transmission of HIV. However, prolonged open-mouth kissing could damage the mouth or lips and allow HIV to pass from an infected person to a partner and then enter the body through cuts or sores in the mouth. Because of this possible risk, the CDC recommends against open-mouth kissing with an infected partner.

Studies have shown that latex condoms are highly effective in preventing HIV transmission when used consistently and correctly. These studies looked at uninfected people considered to be at very high risk of infection because they were involved in sexual relationships with HIV-infected people. The studies found that even with repeated sexual contact, 98-100 percent of those people who used latex condoms correctly and consistently did not become infected.

Yes. Having a sexually transmitted disease (STD) can increase a person's risk of becoming infected with HIV, whether the STD causes open sores or breaks in the skin (e.g., syphilis, herpes, chancroid) or does not cause breaks in the skin (e.g., chlamydia, gonorrhea). If the STD infection causes irritation of the skin, breaks or sores may make it easier for HIV to enter the body during sexual contact.

At the start of every intravenous injection, blood is introduced into needles and syringes. HIV can be found in the blood of a person infected with the virus. The reuse of a blood-contaminated needle or syringe by another drug injector (sometimes called "direct syringe sharing") carries a high risk of HIV transmission because infected blood can be injected directly into the bloodstream. In addition, sharing drug equipment (or "works") can be a risk for spreading HIV.

A risk of HIV transmission does exist if instruments contaminated with blood are either not sterilized or disinfected or are used inappropriately between clients. CDC recommends that instruments that are intended to penetrate the skin be used once, then disposed of or thoroughly cleaned and sterilized.

The only way to determine for sure whether you are infected is to be tested for HIV infection. You cannot rely on symptoms to know whether or not you are infected with HIV. Many people who are infected with HIV do not have any symptoms at all for many years. However, no one should assume they are infected if they have any of these symptoms. Each of these symptoms can be related to other illnesses. Again, the only way to determine whether you are infected is to be tested for HIV infection.

For injection drug users who cannot or will not stop injecting drugs, the following steps may be taken to reduce personal and public health risks: If possible, use sterile water to prepare drugs; otherwise, use clean water from a reliable source (such as fresh tap water). Use a new or disinfected container ("cooker") and a new filter ("cotton") to prepare drugs.

The risk of health care workers getting HIV on the job is very low, especially if they carefully follow universal precautions (i.e., using protective practices and personal protective equipment to prevent HIV and other blood-borne infections). It is important to remember that casual, everyday contact with an HIV-infected person does not expose health care workers or anyone else to HIV.

There are no documented cases of HIV being transmitted during participation in sports. The very low risk of transmission during sports participation would involve sports with direct body contact in which bleeding might be expected to occur. If someone is bleeding, their participation in the sport should be interrupted until the wound stops bleeding and is both antiseptically cleaned and securely bandaged.

When HIV enters your body, it infects your "CD4 cells" and kills them. CD4 cells (sometimes called T-helper cells) help your body fight off infection and disease. Usually, CD4 cell counts in someone with a healthy immune system range from 500 to 1800. When you lose CD4 cells, your immune system breaks down and you can't fight infections and diseases as well. When your CD4 cell count goes under 200, doctors say you have AIDS.

A rapid test for detecting antibody to human immunodeficiency virus (HIV) is a screening test that produces very quick results, in 30 minutes or less. In comparison, results from the commonly used HIV antibody screening test, the enzyme immunoassay (EIA), are not available for 1-2 weeks. Two rapid HIV tests have been approved by the Food and Drug Administration (FDA) for use in the United States. OraQuick Rapid HIV-1 Antibody Test (OraSure Technologies, Inc.

In addition to Pneumocystis Carinii [NEW-mo-SIS-tis CA-RIN-nee-eye] Pneumonia (PCP), you also have a higher chance of getting other diseases, depending on your CD4 count. These are called "opportunistic infections" because a person with HIV can get the infection if his or her weakened immune system gives it the opportunity to develop. More than 100 germs can cause opportunistic infections.