Primary infection

• HIV can enter the body through the sexual organs, the bloodstream and the mouth. Many people experience a flu-like illness a few weeks after being exposed to HIV.
• The 'seroconversion illness' may be accompanied by a rash, ulcers in the mouth, severe headaches, diarrhoea, fevers, aches and pains and swollen glands.
• This illness is followed by the development an immune response to HIV, including the emergence of antibodies, as the immune system brings HIV under control.
• Not everyone experiences such an illness, but within a few months after becoming infected everyone begins to produce antibodies to HIV.
• A small minority of people seems to be at least partially resistant to infection with HIV because of rare genes they have.

How AIDS develops

• The immune system is gradually disrupted. HIV kills cells in the lymph nodes (small glands filled with immune cells that trap foreign organisms) and in other sites. This throws the immune system out of balance.

• Virus levels in the blood and the lymph nodes increase because HIV constantly changes itself to avoid attack by the antibodies and immune cells which normally control infections.

• Each generation of viruses is slightly different, and this constant evolution helps HIV keep one step ahead of the immune system. The immune system cells are always looking for viruses that resemble the previous generation of HIV, so gradually the virus can 'escape' and the viral load increases.

• The CD4 lymphocytes, or T-cells, gradually decline in number because they are killed by HIV, and are killed more rapidly as HIV levels increase.

• HIV destroys the immune system's memory. CD4 T-cells which have been programmed to recognise infections become depleted. This is why opportunistic infections like Pneumocystis pneumonia (PCP) and Candida (thrush) can develop when the CD4 cell count falls low.

Disease progression

• The level of virus in the blood and CD4 cell count can predict the risk of developing AIDS.

• Without treatment of any sort, at least 50% of people infected with HIV will develop AIDS within ten years.

• In the last few years substantial improvements in treatment have reduced the risk of developing AIDS.

• The greater the effect of drugs on virus levels in the body, the lower the risk of developing AIDS in the future.

• No-one knows how long the effects of the new treatments will last, but so far the signs are promising that these effects can be kept up for many years, possibly indefinitely. This is partly because switching to new combinations is possible if a drug combination fails to keep the virus under control.

• Specific co-factors may influence the speed at which HIV disease develops.

• People over 40 develop AIDS faster than younger people.

• Inherited genetic factors may influence the speed at which AIDS develops.

• Lifestyle factors such as vegetarian diet, smoking and drug injecting do not appear to influence disease progression.

• The amount of experience that a doctor has in treating HIV-positive people may influence the risk of death. If a doctor sees a lot of HIV-positive patients the risk of his / her patients dying is lower.

A very small proportion of people infected with HIV show few signs of immune system damage even after being infected for more than ten years. These people are called long-term non-progressors or slow progressors.

Long-term non-progessors have very low virus levels, although the causes of this are unclear. It could be due to a special genetic make-up or the way the immune system responds to HIV.