An opportunistic infection is an infection caused by microorganisms that ordinarily do not produce disease in a healthy host but exploit weakened immune defenses or altered microbiota to cause pathology.
Explanation
Opportunistic infections occur when the host’s normal immune barriers are compromised or when the balance of the resident microbiota is disturbed. Many microbes colonize the skin, mucous membranes and gut without causing harm; others are present in the environment and are held in check by innate and adaptive immunity. Conditions such as HIV infection, neutropenia, organ transplantation, chemotherapy, corticosteroid therapy, malnutrition and advanced age reduce host defenses and allow these organisms to proliferate. Disruption of physical barriers by burns or catheters and broad-spectrum antibiotics that suppress competing flora also create niches for opportunistic pathogens. Unlike primary pathogens, which can cause disease in healthy hosts, opportunistic agents are typically low virulence organisms that rely on host susceptibility for invasion. Their ability to cause infection reflects changes in host-pathogen interactions rather than unique virulence factors.
Common examples and risk factors
Opportunistic infections include fungal, bacterial, viral and protozoan diseases. Candida albicans overgrowth leads to oral thrush and systemic candidiasis in neonates or immunocompromised adults. Pneumocystis jirovecii pneumonia is a hallmark of advanced HIV/AIDS. Cytomegalovirus and herpes simplex virus reactivate when cellular immunity wanes. Bacteria such as Pseudomonas aeruginosa and Acinetobacter species cause severe infections in burn patients and intensive care units. Clostridioides difficile overgrowth occurs after antibiotic therapy, resulting in colitis. Mycobacterium avium complex and Toxoplasma gondii are additional opportunists in people with AIDS. Preventive measures include prophylactic antimicrobials, infection control practices and minimizing immunosuppression when possible.
Opportunistic infections illustrate how changes in host immunity or microbiota can convert benign colonizers into serious pathogens. Recognition of risk factors allows clinicians to monitor and prevent these infections in vulnerable populations.
Related Terms: Immunocompromised, Pathogen, Nosocomial infection, Commensal, Opportunistic pathogen