What Is an Opportunistic Infection?

Opportunistic infections (OIs) are infections that occur more frequently and are more severe in people with weakened immune systems, including people with HIV.

Many OIs are considered AIDS-defining conditions. That means if a person with HIV has one of these conditions, they are diagnosed with AIDS, the most serious stage of HIV infection.

This article will discuss why and how OIs occur in those with HIV, types of OIs, treatment, and prevention.

Tips for Preventing an Opportunistic Infection

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Why Opportunistic Infections Occur

Opportunistic infections are the result of a weakened immune system. In the case of HIV, the virus targets the immune system by depleting CD4 T lymphocytes (CD4 cells). CD4 cells are lymphocytes, a type of white blood cell (WBC) that is vital to helping the body fight off infections by coordinating the immune response.

When an HIV infection occurs, CD4 cells are progressively depleted, leading to immunosuppression and an immunocompromised state. This is why people with HIV are more prone to opportunistic infections and why cases of OIs are more severe in those with HIV.

A normal CD4 cell count is between 500 and 1,500. When immunosuppression occurs, CD4 counts fall below 500. A CD4 count lower than 200 is considered to be AIDS


OIs manifest with disease at different stages of immunosuppression. This means that as a person’s CD4 count plummets, the variety and severity of OIs will increase.

Some of the most common OIs in people living with HIV in the United States are:

  • Herpes simplex virus 1 (HSV-1) infection: A viral infection that can cause sores on the lips and mouth
  • Salmonella infection: A bacterial infection that affects the intestines
  • Candidiasis: A fungal infection of the mouth, bronchi, trachea, lungs, esophagus, or vagina
  • Pneumocystis pneumonia: An infection caused by the fungus Pneumocystis jirovecii

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AIDS-Defining Infection

An AIDS-defining condition is an illness that presents in people who have AIDS.

The AIDS-defining conditions classified by the Centers for Disease Control and Prevention are as follows:

  • Candidiasis
  • Cervical cancer, invasive
  • Coccidioidomycosis, disseminated or extrapulmonary
  • Cryptococcosis, extrapulmonary
  • Cryptosporidiosis, chronic intestinal (more than one month’s duration)
  • Cytomegalovirus (CMV)
  • Encephalopathy (HIV-related)
  • Herpes simplex: chronic ulcers (lasting longer than one month) or bronchitis, pneumonitis, or esophagitis
  • Histoplasmosis, disseminated or extrapulmonary
  • Isosporiasis, chronic intestinal (lasting longer than one month)
  • Kaposi sarcoma
  • Lymphoid interstitial pneumonia or pulmonary lymphoid hyperplasia complex
  • Burkitt lymphoma
  • Immunoblastic lymphoma
  • Primary lymphoma of the brain
  • Mycobacterium avium complex
  • Pneumocystis pneumonia
  • Pneumonia
  • Progressive multifocal leukoencephalopathy
  • Salmonella septicemia
  • Toxoplasmosis
  • Wasting syndrome


Treatment for each type of OI depends on what is causing the infection.

Antiretroviral therapy (ART) is medication that reduces the amount of HIV in the blood. The amount of HIV in the blood is referred to as the viral load. While ART does not cure HIV, it does make the condition incredibly manageable, helping those with HIV live longer, healthier lives.

A study funded by the National Institutes of Health concluded that ART can help improve or restore the CD4 count, improving the overall health of people with HIV. ART can also make the viral load low enough to be undetectable, meaning you can no longer spread the virus to a partner via sex. This is called an undetectable viral load.

The study also showed that the earlier ART is started, the better. Incidences of serious AIDS-related events and serious non-AIDS-related events were both lower in the early treatment group compared to the deferred treatment group.


If you have HIV, there are a number of ways to prevent OIs. Some such strategies for avoiding opportunistic infections when you have HIV include:

  • Avoid exposure to contaminated food and water.
  • Take medication to prevent certain OIs.
  • Stay up-to-date on vaccinations.
  • Travel safely.
  • Prevent exposure to other sexually transmitted diseases.

The best way to avoid OIs, however, when you have HIV is to take your HIV medication as directed, as ART keeps your immune system strong and healthy.

A Word From Verywell

HIV can be a frightening diagnosis to receive. However, HIV is no longer the death sentence it was in the 1980s and early 1990s due to advances in treatment, specifically antiretroviral therapy.

With dedicated adherence to ART, people with HIV can live a normal to near-normal life, including in regard to life expectancies.

If you have HIV and have any concerns, be sure to ask questions of your doctor or healthcare provider.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. HIV.gov. What are opportunistic infections?

  2. Centers for Disease Control and Prevention. AIDS and opportunistic infections.

  3. Centers for Disease Control and Prevention. HIV treatment.

  4. National Institutes of Health. Starting antiretroviral treatment early improves outcomes for HIV-infected individuals.

  5. Lundgren JD, Babiker AG, Gordin F, et al. Initiation of antiretroviral therapy in early asymptomatic HIV infection. N Engl J Med. 2015;373(9):795-807. doi:10.1056/NEJMoa1506816

By Molly Burford
Molly Burford is a mental health advocate and wellness book author with almost 10 years of experience in digital media.