Opportunistic Infection

CANGE, HAITI - MARCH 24: Blood tests wait to be inspected at the lab of Zanmi Lasante Hospital March 24, 2005 in Cange, Haiti. Many HIV positive patients come to be hospitalized here, but the majority of HIV infected people will stay at home in their final stage of life and will die there as most hospitals in the country can not take them. (Photo by Shaul Schwarz/Getty Images). Shaul Schwarz/Getty Images

What is the Definition of Opportunistic Infection

 An infection is said to be "opportunistic" when it takes advantage of the fact that patients are immunocompromised. Opportunistic infections are infections that are pretty easy for a healthy immune system to fight off. Thus, opportunistic infections only appear in people whose immune system is compromised. This can happen due to illness, age, or other factors.

Opportunistic infections are caused by organisms that may be present in healthy people but do not normally cause disease in them. However, when these same organisms infect a person who is immunocompromised, there's a problem. Their bodies can not fight off the infection and they become sick.

Opportunistic infections are one of the hallmarks of AIDS. In fact, the presence of opportunistic infections is one of the defining characteristics of AIDS. However, opportunistic infections do not only occur in AIDS patients. They can occur in anyone who is immune deficient.

How Opportunistic Infections Led To The Discovery of AIDS

A rise in unusual opportunistic infections was one of the things that led to the discovery of HIV and AIDS. All of a sudden there was a rise in the appearance of previously rare diseases -- such as Pneumocystis jirovecii pneumonia. Trying to figure out what the people with those diseases had in common helped early HIV researchers understand that a new type of contagious disease was on the rise.

They saw the effects of the infection. Then they had to search for the cause.

How did they discover that this acquired immune deficiency was genetic? A variety of these opportunistic infections, which were rarely seen in the general population, started showing up in populations connected by sexual networks and other infectious ties.

This led doctors to look for a pathogen that could be affecting people's immune systems. Eventually, they discovered AIDS.

Common Opportunistic Infections in Untreated HIV

When properly treated, people with HIV can live long healthy lives. They may never become immunocompromised or develop an opportunistic infection. However often HIV goes undiagnosed for a period of time. People aren't tested regularly and don't know they're infected. As such there are still some opportunistic infections that are relatively common in the United States. The specific diseases that are seen most often vary by area, but some relatively widespread ones include:

  • Thrush, or other forms of candidiasis
  • Cryptococcal meningitis and pneumonia
  • Pneumocystis carinii pneumonia (PCP) (now pneumocystis jirovecii pneumonia)
  • Invasive cervical cancer
  • Coccidioidomycosis
  • Cryptosporidium diarrhea
  • CMV infection
  • Chronic herpes ulcers, that last a month or more at a time
  • Kaposi's sarcoma
  • Tuberculosis
  • Toxoplasmosis

The presence of such infections is one of the defining characteristics of AIDS.

Someone can also be determined to have AIDS if they have a very low CD4 count


AIDS.gov (2011) A Timeline of AIDS. Accessed 1/5/16 from  https://www.aids.gov/hiv-aids-basics/hiv-aids-101/aids-timeline/

CDC (2015) Opportunistic Infections. Accessed 1/5/16 at  http://www.cdc.gov/hiv/basics/livingwithhiv/opportunisticinfections.html

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