Opportunistic Infections

An opportunistic infection is one caused by a microorganism that ordinarily does not produce disease in a person with intact defenses, but exploits the opportunity created by impaired immunity, breached barriers, or altered microbial flora. The concept ties the identity of the infecting organism to the specific weakness in the host.

Definition

An opportunistic infection is an infection caused by a normally commensal or low-virulence organism that produces disease when host defenses are compromised by immunodeficiency, mucosal or skin barrier disruption, indwelling devices, or disturbance of the normal microbiota.

Scope

This topic covers what defines an opportunistic infection, the host conditions that permit it, and the way the type of immune defect predicts the responsible pathogens, with examples drawn from HIV/AIDS, transplantation, and other immunosuppressed states. It is a reference description of a clinical category rather than a management protocol.

Core questions

• What makes an infection opportunistic rather than conventional?
• Which host defects predispose to which opportunistic pathogens?
• How does restoring host defense change the risk and course of these infections?

Key concepts

• Opportunistic pathogen
• Net state of immunosuppression
• T-cell versus neutrophil versus humoral defects
• Commensal organisms as pathogens
• Immune reconstitution and risk reversal
• Latent infection reactivation

Mechanisms

Opportunistic infection arises when the balance between microbe and host shifts in the microbe's favour. Defects in cell-mediated immunity (for example, low CD4 counts in advanced HIV) predispose to intracellular and fungal pathogens such as Pneumocystis, mycobacteria, and cytomegalovirus; neutropenia predisposes to bacterial and mould infections; and humoral or splenic defects predispose to encapsulated bacteria. The composite of these factors, sometimes summarised as the net state of immunosuppression, together with epidemiologic exposures, determines which organisms cause disease in a given patient (Fishman, 2007). Many opportunistic infections represent reactivation of latent organisms that were previously contained by an intact immune response (Aberg et al., 2013).

Clinical relevance

Recognising the pattern of immune deficiency helps frame which opportunistic infections are plausible and underlies the rationale for prophylaxis in selected high-risk groups. This entry explains the category and its logic; it is educational and does not provide diagnostic thresholds, drug choices, or individualised treatment guidance.

Epidemiology

Opportunistic infections became prominent in clinical medicine with the AIDS epidemic, where the depth of CD4 depletion strongly predicted specific infections, and with the expansion of transplantation and immunosuppressive therapy. Effective antiretroviral therapy and structured prophylaxis have markedly reduced the incidence of several classic AIDS-defining opportunistic infections (Aberg et al., 2013).

History

The term gained clinical weight in the latter twentieth century as immunosuppressive medicine and then the AIDS pandemic revealed how organisms long regarded as harmless commensals could become lethal once host defenses failed. Studies of complications such as progressive multifocal leukoencephalopathy in HIV illustrated how profound immunosuppression unmasks otherwise quiescent pathogens (Hall et al., 1998).

Key figures

• Jay Fishman
• Judith Aberg

Seminal works

• fishman-2007
• aberg-2013

Frequently asked questions

What is an opportunistic infection?

It is an infection caused by an organism that usually does not cause disease in healthy people but takes advantage of weakened host defenses. The same microbe that is harmless to most people can cause serious illness when immunity is impaired.

Why do different immune defects cause different infections?

Each arm of the immune system controls particular kinds of organisms, so the type of defect shapes the risk. For example, defects in cell-mediated immunity favour intracellular and fungal pathogens, while loss of neutrophils favours certain bacteria and moulds.