In the context of infectious disease, what is an opportunistic infection?

• A. An infection that results from ingestion of a preformed toxin rather than microbial growth in the host.
• B. An infection caused by a pathogen that is always able to cause disease in healthy individuals and does not depend on host immune status.
• C. An infection caused by microorganisms that usually do not cause disease in healthy hosts but cause disease when host defenses are compromised (e.g., immunosuppression or disruption of normal microbiota).
• D. An infection acquired exclusively through direct contact with an infected person, requiring person-to-person transmission.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the definition of an opportunistic infection. It refers to infections caused by microorganisms that typically do not cause disease in healthy individuals but can cause disease when the host's immune defenses are weakened or compromised.
2. Step 2: Recognize that opportunistic infections often occur in situations such as immunosuppression (e.g., HIV/AIDS, chemotherapy), disruption of normal microbiota (e.g., after antibiotic use), or other conditions that impair the immune system.
3. Step 3: Differentiate opportunistic infections from infections caused by preformed toxins, which result from ingestion of toxins rather than microbial growth within the host.
4. Step 4: Distinguish opportunistic infections from infections caused by primary pathogens that can cause disease in healthy individuals regardless of immune status.
5. Step 5: Note that opportunistic infections are not defined by the mode of transmission (such as person-to-person contact) but by the host's compromised immune state allowing normally harmless microbes to cause disease.