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Opportunistic infection

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Opportunistic infection
Chest X-ray of a patient who first had influenza and then developed Haemophilus influenzae pneumonia, presumably opportunistic
SpecialtyInfectious diseases Edit this on Wikidata

An opportunistic infection is an infection caused by pathogens (bacteria, fungi, parasites or viruses) that take advantage of an opportunity not normally available. These opportunities can stem from a variety of sources, such as a weakened immune system (as can occur in acquired immunodeficiency syndrome or when being treated with immunosuppressive drugs, as in cancer treatment),[1] an altered microbiome (such as a disruption in gut microbiota), or breached integumentary barriers (as in penetrating trauma). Many of these pathogens do not necessarily cause disease in a healthy host that has a non-compromised immune system, and can, in some cases, act as commensals until the balance of the immune system is disrupted.[2][3] Opportunistic infections can also be attributed to pathogens which cause mild illness in healthy individuals but lead to more serious illness when given the opportunity to take advantage of an immunocompromised host.[4]

Types of opportunistic infections[edit]

A wide variety of pathogens are involved in opportunistic infection and can cause a similarly wide range in pathologies. A partial list of opportunistic pathogens and their associated presentations includes:






Immunodeficiency or immunosuppression are characterized by the absence of or disruption in components of the immune system, leading to lower-than-normal levels of immune function and immunity against pathogens.[1] They can be caused by a variety of factors, including:

The lack of or the disruption of normal vaginal microbiota allows the proliferation of opportunistic microorganisms and will cause the opportunistic infection bacterial vaginosis.[38][39][40][41]

Opportunistic Infection and HIV/AIDS[edit]

HIV is a virus that targets T cells of the immune system and, as a result, HIV infection can lead to progressively worsening immunodeficiency, a condition ideal for the development of opportunistic infection.[42][43] Because of this, respiratory and central nervous system opportunistic infections, including tuberculosis and meningitis, respectively, are associated with later-stage HIV infection, as are numerous other infectious pathologies.[44][45] Kaposi's sarcoma, a virally-associated cancer, has higher incidence rates in HIV-positive patients than in the general population.[46] As immune function declines and HIV-infection progresses to AIDS, individuals are at an increased risk of opportunistic infections that their immune systems are no longer capable of responding properly to. Because of this, opportunistic infections are a leading cause of HIV/AIDS-related deaths.[47]


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.[48]

Restoration of immune system[edit]

  • 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]

Avoidance of infectious exposure[edit]

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

Prophylactic medications[edit]

Individuals at higher risk are often prescribed prophylactic medication to prevent an infection from occurring. A person's risk level for developing an opportunistic infection is approximated using the person's CD4 T-cell count and other indications. The table below provides information regarding the treatment management of common opportunistic infections.[54][55][56]

Opportunistic infections Indication(s) for prophylactic medications Preferred agent(s) When to discontinue agent(s) Secondary prophylactic/maintenance agent(s)
Mycobacterium tuberculosis Upon diagnosis of HIV, any positive screening test, or prior medical history of Mycobacterium tuberculosis. These current agents' doses/frequency will discontinue after two months. Depending on clinical presentation, maintenance agents will continue for at least four more months.
  • Rifampicin, isoniazid, and pyridoxine
Pneumocystis jiroveci CD4 count is less than 200 cells/mm3 or less than 14%. The person has documented medical history of recurrent oropharyngeal candidiasis. This current agent doses/frequency will discontinue after 21 days. Secondary prophylactic agent dose/frequency will continue until the CD4 count is above 200 cells/mm3 and the HIV viral load is undetectable for at least three months while taking antiretroviral therapy.
  • Trimethoprim-sulfamethoxazole
Toxoplasma gondii CD4 count is less than 100 cells/mm3 or less than 14%, and the person has a positive serology for Toxoplasma gondii.
  • Trimethoprim-sulfamethoxazole
This agent will discontinue after six weeks. Secondary prophylactic medications will continue until the CD4 count is above 200 cells/mm3 and HIV viral load is undetectable for at least six months while taking antiretroviral therapy.
Mycobacterium avium complex disease CD4 count is less than 50 cells/mm3 and has a detectable viral load while taking antiretroviral therapy.
  • Clarithromycin and ethambutol
  • Rifabutin may be added depending on clinical presentation.
These agent(s) will discontinue after 12 months only if the person does not have any symptoms that will be concerning for persistent Mycobacterium avium complex disease and their CD4 count is above 100 cells/mm3, and while their HIV viral load is undetectable for at least six months while taking antiretroviral therapy. N/A

Alternative agents can be used instead of the preferred agents. These alternative agents may be used due to allergies, availability, or clinical presentation. The alternative agents are listed in the table below.[54][55][56]

Opportunistic infections Alternative agent(s)
Mycobacterium tuberculosis
Pneumocystis jiroveci
Toxoplasma gondii
  • Dapsone, pyrimethamine, and folinic acid
  • Atovaquone, pyrimethamine, and folinic acid
Mycobacterium avium complex disease


Treatment depends on the type of opportunistic infection, but usually involves different antibiotics.[citation needed]

Veterinary treatment[edit]

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


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