Opportunistic infection

• Clostridioides difficile (formerly known as Clostridium difficile) is a species of bacteria that is known to cause gastrointestinal infection and is typically associated with the hospital setting.[5][6]
• Legionella pneumophila is a bacterium that causes Legionnaire’s disease, a respiratory infection.[7][8]
• Mycobacterium avium complex (MAC) is a group of two bacteria, M. avium and M. intracellulare, that typically co-infect, leading to a lung infection called mycobacterium avium-intracellulare infection.[9][10]
• Mycobacterium tuberculosis is a species of bacteria that causes tuberculosis, a respiratory infection.[11]
• Pseudomonas aeruginosa is a bacterium that can cause respiratory infections. It is frequently associated with cystic fibrosis and hospital-acquired infections.[12]
• Salmonella is a genus of bacteria, known to cause gastrointestinal infections.[13]
• Staphylococcus aureus is a bacterium known to cause skin infections and sepsis, among other pathologies. Notably, S. aureus has evolved several drug-resistant strains, including MRSA.[14][15]
• Streptococcus pneumoniae is a bacterium that causes respiratory infections.[2]
• Streptococcus pyogenes (also known as group A Streptococcus) is a bacterium that can cause a variety of pathologies, including impetigo and strep throat, as well as other, more serious, illnesses.[16][17]

• Aspergillus is a fungus, commonly associated with respiratory infection.[18][19]
• Candida albicans is a species of fungus that is associated with oral thrush and gastrointestinal infection.[20][21]
• Coccidioides immitis is a fungus known for causing coccidioidomycosis, more commonly known as Valley Fever.[22]
• Cryptococcus neoformans is a fungus that causes cryptococcosis, which can lead to pulmonary infection as well as nervous system infections, like meningitis.[23][24]
• Histoplasma capsulatum is a species of fungus known to cause histoplasmosis, which can present with an array of symptoms, but often involves respiratory infection.[25][26]
• Pseudogymnoascus destructans (formerly known as Geomyces destructans) is a fungus that causes white-nose syndrome in bats.[27]
• Microsporidia is a group of fungi that infect species across the animal kingdom, one species of which can cause microsporidiosis in immunocompromised human hosts.[28]
• Pneumocystis jirovecii (formerly known as Pneumocystis carinii) is a fungus that causes pneumocystis pneumonia, a respiratory infection.[29]

• Cryptosporidium is a protozoan that infects the gastrointestinal tract.[30]
• Toxoplasma gondii is a protozoan, known for causing toxoplasmosis.[31][32]

• Cytomegalovirus is a family of opportunistic viruses, most frequently associated with respiratory infection.[19][33]
• Human polyomavirus 2 (also known as JC virus) is known to cause progressive multifocal leukoencephalopathy (PML).[34][35]
• Human herpesvirus 8 (also known as Kaposi sarcoma-associated herpesvirus) is a virus associated with Kaposi sarcoma, a type of cancer.[36][37]

• In patients with HIV, starting antiretroviral therapy is especially important for restoration of the immune system and reducing the incidence rate of opportunistic infections[49][50]
• 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.[51][52]

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.[53]
• 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.

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:[53]