An opportunistic infection is an infection caused by a normally benign microorganism which has become pathogenic. Opportunistic infections occur in people with compromised immune systems which allow such organisms to take over and cause widespread infection. In individuals with healthy immune system, these organisms would never be allowed to spread to the point of causing infection, because the immune system would keep them at bay.
A number of people are at risk for opportunistic infection, also known as OI. The classic example is HIV/AIDS patients, infected with a retrovirus which essentially shuts down the immune system. People undergoing chemotherapy for cancer are also at risk, as are individuals taking drugs to suppress the immune system in preparation for organ transplant, victims of malnutrition, and people with existing infections, especially the elderly.
Some of the microorganisms which cause opportunistic infections are actually already present in the body. Organisms like cytomegalovirus are present in well over 50% of the population, for example. People with compromised immune systems can experience an opportunistic infection as fungi, bacteria, and protozoans in their own bodies run rampant, or as a result of exposure to organisms carried by other people or animals. An opportunistic infection can also occur when a normally mildly virulent microorganism enters the body, which is why people with fevers, coughs, and colds are asked to stay away from people with compromised immune systems.
One way to combat opportunistic infection is to take prophylactic drugs which are designed to make the body hostile to harmful invaders. However, it is impossible to guard against all potential sources of infection, and in some regions, people may not be able to pay for prophylactic therapy. Therefore, it is important for people with compromised immune systems to receive regular medical checkups, so that early signs of infection can be spotted before the situation becomes serious.
Once an opportunistic infection is identified, it must be treated so that it cannot spread and cause more damage. However, treatment is complicated by the patient's existing medical condition. For example, a cancer called Kaposi's Sarcoma develops in some AIDS patients. Under normal conditions, the cancer is benign, but it can be treated with chemotherapy. Chemotherapy, however, would destroy the immune system of the patient, so other treatment approaches must be used. Sometimes, no effective treatment is available, which is why opportunistic infections are so dangerous.