Contrary to some popular misconceptions, HIV is a difficult disease to get. Below we talk about the ways HIV is spread -- and how it is not spread.

Three conditions must be met for HIV transmission to occur:

HIV must be present;

Infection can only happen if one of the persons involved is infected with HIV. Some people assume that certain behaviors (such as anal sex) cause AIDS, even if HIV is not present. This is not true.

In sufficient quantity;

The concentration of HIV determines whether infection may happen. In blood, for example, the virus is very concentrated. A small amount of blood is enough to infect someone. A much larger amount of other fluids would be needed for HIV transmission.

And it must get into the bloodstream;

It is not enough to be in contact with an infected fluid to become infected. Healthy, unbroken skin does not allow HIV to get into the body; it is an excellent barrier to HIV infection. HIV can only enter through an open cut or sore, or through contact with the mucous membranes in the anus and rectum, the genitals, the mouth, and the eyes.

Generally, when people ask the question, "How long can HIV survive outside the body?" they have come into contact with some body fluid that they think might contain HIV, and are worried about transmission. Almost always these questions are about casual contact, and we know the virus is not transmitted except during unprotected sex, sharing needles, or through significant and direct exposure to infected blood.

Length of time

The length of time HIV can survive outside the body depends on:

• the amount of HIV present in the body fluid;
• what conditions the fluid is subjected to

In a laboratory, HIV has been kept viable (able to infect) for up to 15 days, and even after the body fluid containing it had dried. However, these experiments involved an extremely high concentration of the virus which was kept at a stable temperature and humidity. These conditions are very unlikely to exist outside of a laboratory. HIV is very fragile, and many common substances, including hot water, soap, bleach and alcohol, will kill it.

A study of over 2,000 health care workers has been underway for several years to assess the risk of their exposure to people with AIDS. Over 1,000 of these workers had a needle stick accident with a needle that had been used on a person living with AIDS. The rest had some sort of mucous membrane exposure, such as being splashed in the face with blood or vomit.

Of all these people, only 21 show signs of being infected with HIV (as determined by the antibody test). One of these people was a nurse who had multiple needle stick accidents, including one where she tripped and fell on the depressor of a syringe full of blood, and the entire contents entered her body. Another was a lab worker who was working with a test tube of infected blood which broke and cut his finger, exposing the infected blood to his bloodstream. This study shows that AIDS is a difficult disease to get, and even the intimate exposure of these health care workers was not enough to infect them, except in the most extreme cases.

Since March 1985, all blood has been screened with the HIV antibody test. This practice has almost eliminated the risk of getting HIV through a blood transfusion.

Hemophilia is a genetic disease in which people (almost all men) lack the ability to clot blood. To control the condition, hemophiliacs take Factor VIII, a clotting factor. Each dose of Factor VIII comes from the pooled blood of many donors.

Besides whole blood, platelets (red blood cells) have transmitted the virus. Current blood screening, however, should prevent all but a very, very few cases. No other blood products are suspected of transmitting HIV. Gamma globulin or hepatitis B vaccine do not transmit HIV. Gamma globulin, however, can temporarily transmit HIV antibodies, although not the virus itself. These antibodies will disappear within a few months.

Donor semen is checked for HIV antibodies when the semen is collected. The semen is then frozen. The donor is required to come back after six months for a second HIV test, to confirm the initial HIV screening. The semen is not used before the procedure is completed.