HIV can only be transmitted and cause infection if it gets into the body via a route where vulnerable cells are plentiful.

On a cellular level, HIV can infect CD4 cells and CD8 cells in the blood as well as cells in the lungs, the brain, the gastrointestinal tract and the kidneys. But to reach these cells, HIV must pass through cells in the mucous membranes called dendritic cells and Langerhans cells.

Dendritic cells are in fact designed precisely to ‘ferry’ infectious particles to the lymph nodes where they act as antigens that stimulate the immune response; it is exactly this otherwise protective activity that HIV exploits in order to achieve infection.

Dendritic cells do not become infected with HIV themselves but the virus’s gp120 envelope protein attaches itself to a ‘sticky’ molecule called DC-SIGN which coats the cells (see Geijtenbeek 2000).

Therefore, blood splashed onto the skin will not come into contact with the Langerhans cells beneath the skin because these are not exposed. The skin must be damaged, lacerated or deeply wounded for this to happen. In the rectum and vagina by contrast, infected blood or semen will reach the Langerhans cells far more easily because they are close to the surface and lack the covering of the thick dead epithelial layer which is the dry surface of the skin.

The effective routes for transmission to occur are:

• Directly into the blood (parenterally) through a wound; Through a cut or sore or damaged skin; An injection using contaminated and unsterilised injection equipment; An invasive surgical procedure such as an organ transplant or blood transfusion
• Through particular 'interior' sections of skin called mucous membranes in the rectum, the vagina and cervix; the urethra (which is in front of the vagina in women and the tube of the penis in men); mucous membranes in the eyes and mouth in rare circumstances.

It should be noted that in these cases, trauma or damage to the tissue is not always necessary for infection to occur. These mucous membranes will allow HIV to be absorbed into cells which facilitate infection. In the early years of the epidemic it was thought that damage to these tissues might be the precondition of infection. We now know that infection occurs without such damage, although damage will certainly increase the chances of infection.

There are only four proven, substantial routes of transmission:

1. Unprotected intercourse with someone who is infected.
2. Sharing unsterilised injection equipment which has been previously used by someone who is infected.
3. Injection or transfusion of contaminated blood or blood products, and donations of semen (artificial insemination), skin grafts and organ transplants taken from someone who is infected.
4. From a mother who is infected to her baby (this may be during the course of pregnancy, at birth and through breastfeeding).