An opportunistic infection is an infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system, an altered microbiota (such as a disrupted gut microbiota), or breached integumentary barriers. Many of these pathogens do not cause disease in a healthy host that has a normal immune system. However, a compromised immune system, which is seriously debilitated and has lowered resistance to infection, a penetrating injury, or a lack of competition from normal commensals presents an opportunity for the pathogen to infect.

Types of infections

A partial listing of opportunistic organisms includes Aspergillus, Candida albicans, Clostridium difficile, Coccidioides immitis, Cryptococcus neoformans, Cryptosporidium, Cytomegalovirus, Geomyces destructans (bats), Histoplasma capsulatum, Isospora belli, Polyomavirus JC polyomavirus, the virus that causes Progressive multifocal leukoencephalopathy, Kaposi's Sarcoma caused by Human herpesvirus 8 (HHV8), also called Kaposi's sarcoma-associated herpesvirus (KSHV), Legionnaires' Disease (Legionella pneumophila), Microsporidium.

Causes

Immunodeficiency or immunosuppression can be caused by Malnutrition, Fatigue, Recurrent infections, Immunosuppressing agents for organ transplant recipients, Advanced HIV infection, Chemotherapy for cancer, Genetic predisposition, Skin damage, Antibiotic treatment.

Prevention

Since opportunistic infections can cause severe disease, much emphasis is placed on measures to prevent infection. Such a strategy usually includes restoration of the immune system as soon as possible, avoiding exposures to infectious agents, and using antimicrobial medications ("prophylactic medications") directed against specific infections.

Restoration of immune system

• In patients with HIV, starting antiretroviral therapy is especially important for restoration of the immune system and reducing the incidence rate of opportunistic infections
• In patients undergoing chemotherapy, completion of and recovery from treatment is the primary method for immune system restoration. In a select subset of high risk patients, granulocyte colony stimulating factors (G-CSF) can be used to aid immune system recovery.

Avoidance of infectious exposure

The following may be avoided as a preventative measure to reduce risk of infection:

• Eating undercooked meat or eggs, unpasteurized dairy products or juices
• Potential sources of tuberculosis (high risk healthcare facilities, regions with high rates of tuberculosis, patients with known tuberculosis)
• Any oral exposure to feces.
• Contact with farm animals, especially those with diarrhea: source of Toxoplasma gondii, Cryptosporidium parvum
• Cat feces (e.g. cat litter): source of Toxoplasma gondii, Bartonella spp.
• Soil/dust in areas where there is known histoplasmosis, coccidiomycosis
• Reptiles, chicks, and ducklings that are a common source of Salmonella.
• Unprotected sexual intercourse with individuals with known sexually transmitted infections.

Prophylactic medications

Individuals at higher risk are often prescribed prophylactic medication to prevent an infection from occurring. A patient's risk level for developing an opportunistic infection is approximated using the patient's CD4 T-cell count.