Opportunistic Infection

An opportunistic infection (from Latin opportunus — convenient, favorable) is a disease caused by microorganisms that normally coexist harmlessly with humans or remain dormant but acquire aggressive pathogenic properties in the presence of profound immunodeficiency (reduced human immune defenses).

Etiology and pathophysiology

The skin, mucous membranes, and intestine are colonized by a substantial number of diverse microbes—bacteria, viruses, protozoa, and fungi. A number of latent herpesviruses (e.g., cytomegalovirus and Epstein–Barr virus) also persist in the body for life.

In a healthy person, robust T-cell immunity keeps these “neighbors” under control, preventing them from penetrating the internal organs and causing disease.

The pathogenesis of an opportunistic infection is initiated when this control is disrupted (severe T-cell immunodeficiency). This occurs in the following situations:

• at the AIDS stage — in HIV infection;
• during treatment with potent immunosuppressants — after donor organ transplantation;
• during aggressive chemotherapy — in the treatment of cancer.

Once deprived of the restraining factor, conditionally pathogenic flora begin to proliferate uncontrollably, breach tissue barriers, enter the systemic circulation, and extensively affect the lungs, brain, gastrointestinal tract, and other organs.

Clinical significance

Opportunistic infections are a common cause of mortality among immunocompromised patients. Classic examples of such disorders include:

• Pneumocystis pneumonia;
• candidal esophagitis;
• cerebral toxoplasmosis;
• cytomegalovirus retinitis — inflammation of the retina leading to blindness;
• colitis.

Such infections are often a direct clinical indication for HIV testing.

The diagnosis and treatment of opportunistic infections constitute a complex medical challenge: they require the administration of antimicrobial drugs in combination with treatment of the underlying cause of immunosuppression to restore the patient’s immunity.