Opportunistic infection

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 sp.
 * 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
 * Mycobacterium avium complex (MAC) (Nontuberculosis Mycobacterium)
 * Mycobacterium tuberculosis
 * Pneumocystis jirovecii, previously known as Pneumocystis carinii f. hominis
 * Pseudomonas aeruginosa
 * Salmonella
 * Staphylococcus aureus
 * Streptococcus pneumoniae
 * Streptococcus pyogenes
 * Toxoplasma gondii

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 leading to disruption of the physiological microbiome, thus allowing some microorganisms to outcompete others and become pathogenic (e.g. disruption of intestinal microbiota may lead to Clostridium difficile infection
 * Medical procedures
 * Pregnancy
 * Ageing
 * Leukopenia (i.e. neutropenia and lymphocytopenia)

The lack of or the disruption of normal vaginal microbiota allows the proliferation of opportunistic microorganisms and will cause the opportunistic infection - bacterial vaginosis.

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 reduces the incidence 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.

Infectious exposures to avoid

 * Cat feces (e.g. cat litter): source of Toxoplasma gondii, Bartonella spp.
 * 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)
 * Contact with farm animals, especially those with diarrhea: source of Toxoplasma gondii, Cryptosporidium parvum
 * Soil/dust in areas where there is known histoplasmosis, coccidiomycosis
 * Reptiles, chicks, ducklings: source of Salmonella spp.
 * Unprotected sexual intercourse with individuals with known sexually transmitted infections.
 * Any oral exposure to feces.

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 and sometimes other markers of susceptibility. Common prophylaxis treatments include the following:

Treatment
Treatment depends on the type of opportunistic infection, but usually involves different antibiotics.

Veterinary treatment
Opportunistic infections caused by feline leukemia virus and feline immunodeficiency virus retroviral infections can be treated with lymphocyte T-cell immunomodulator.