One of the most important components of an HIV treatment strategy is the treatment and prevention of opportunistic infections.

Opportunistic infections (OIs) are infections that take advantage of HIV-related damage to the immune system. Most OIs are caused by micro-organisms that are common and may have lived in the body for many years. While the immune system is working properly, these micro-organisms are prevented from growing or spreading within the body and causing disease. However, if the immune system is damaged by HIV (or other sources of immune suppression such as post-transplant treatment drugs) the micro-organisms are able to reactivate and proliferate.

Anti-HIV therapy is now considered the best way to prevent opportunistic infections. For people with low CD4 counts (below 250 cells/mm³), other medicines may be prescribed to prevent specific opportunistic infections. In many cases, OIs can be effectively treated when they occur, although there are several for which adequate treatments do not exist.

In some cases it may be possible to avoid becoming infected with the organisms which cause illness. Taking care with food and drink, pet handling and safer sex and safer drug use may all prevent infection with potentially harmful organisms. However, these common sense precautions cannot protect against the reactivation of micro-organisms which may have been acquired a long time ago.

Treatments to prevent the development of an infection are known as prophylaxis, from the Greek word meaning 'to guard against'. Primary prophylaxis simply means trying to prevent even a first occurrence of an opportunistic infection in someone who is likely to be vulnerable to it. It is quite well established that particular opportunistic infections usually only occur after a particular degree of immune damage has taken place, as measured by the CD4 cell count. This makes it possible to start particular prophylactic drugs when the CD4 cell count falls below certain levels.

Secondary prophylaxis means trying to prevent a subsequent episode of an opportunistic infection after one episode has occurred. It is standard medical practice to provide 'maintenance therapy' to people who have already experienced certain opportunistic infections, such as tuberculosis, cytomegalovirus of the eye or brain, toxoplasmosis or cryptococcal infection, to prevent a recurrence. Maintenance therapy usually consists of lower doses of similar drugs to those used to treat the active infection.

Making a choice on primary or secondary prophylaxis is an individual decision made between people with HIV and their doctor. Targeting prophylaxis is also important. No-one who feels entirely healthy will want to start taking prophylactic drugs until he or she is genuinely at risk. There is also a chance of resistance to drugs occurring if pathogens are exposed to those drugs over long periods. The choice of drug is also clearly important: some treatments for active opportunistic infections are too toxic to be given over the long periods needed for prophylaxis.

Does prophylaxis work?

There is good evidence that prophylaxis works. For example, the incidence of PCP as a first AIDS diagnosis decreased in the mid 1990s, before the widespread uptake of antiretroviral therapy, due to primary prophylaxis. Studies have shown that prophylaxis and antiretroviral therapy has increased survival among people with HIV. See Survival, AIDS and mortality in the section Anti-HIV therapy: Effectiveness of HIV therapy for more details.

In the 2000s, people with HIV who develop AIDS tend to do so only when their CD4 count has fallen to a much lower level than in the past. This is presumably because antiviral treatment and the use of prophylaxis are delaying the opportunistic diseases that might otherwise occur earlier. Unfortunately prophylaxis is not always as effective once the CD4 count becomes very low, although it remains of value.

For further discussion of prophylaxis see Immune restoration illnesses and Prophylaxis and immune recovery in Anti-HIV therapy: Restoring the immune system.

Guidelines on when to use prophylaxis can be found in United States Public Health Service guidelines on prevention of opportunistic infections in Anti-HIV therapy: Guidelines.

For further information on prophylaxis in children see Treatment options in Anti-HIV therapy: Options for children.