HIV is a simple virus. The most important thing to remember is that it cannot survive outside the body at all. But inside a human a body, conditions are perfect for the virus to survive and multiply. Why?
Body temperature is ideal. High temperatures cause the virus to die quickly. Anything higher than 50 degrees C is enough to destroy the virus. The moistness of the body is ideal, outside the body the virus will die. No contact with air, this is the most important one. The outer protein layer of the HI virus oxidizes on coming into contact with air and it therefore dies very quickly. In the body it does not come into contact with air and consequently survives. The correct pH balance plays a vital role, as the virus needs a pH balance very close to that which we find within the human body. As soon as the environment changes to being too acidic or alkaline, the virus would die.
During sex, all these conditions are present together with the body fluids that can carry high concentrations of HIV. This is why so many people are infected during unprotected sex.
There are some conditions or situations that will make a person more vulnerable to becoming HIV infected. First is the entry point. There must be a point for the virus to enter the body. People very often think only in terms of an open wound, cut or injection needle injury. Yes, if there were direct contact of infected, high concentration body fluids into that cut, there would be a definite risk of infection. But the point of entry could also occur during sexual intercourse, through a small lesion occurring in the mucous membranes of the sexual organs.
The presence of Sexually Transmitted Infections or STIs is also a risk-increasing condition. Most STIs cause small sores or small areas of broken skin on the sexual organs or in the mouth. This creates an ideal place for the virus to enter the bloodstream. There is therefore a very strong correlation between the occurrence of STIs and HIV.
The higher the concentration of the virus or the greater the quantity of viruses present, the easier it is to be infected. The lower the concentration, the smaller the chance of infection. This applies both to the concentration of the virus due to the stage of disease progression the HIV infected person might be in. If you remember, the level fluctuates during the course of the disease. Just after infection and in the fourth phase, there is a very high concentration of the virus. Your chance of becoming infected during these phases as a result of direct contact with high concentration body fluids are therefore greater than in other phases.
Note: Information was excerpt from the "Channels of Hope" manual.