What are opportunistic infections?

• A. Illnesses caused by organisms that usually do not cause disease in people with normal immune systems.
• B. Infections that occur only after organ transplant.
• C. Infections caused by any virus during pregnancy.
• D. Infections that always cause severe illness in healthy individuals.

Answer: (A)

Opportunistic infections occur when the immune system is weakened and can no longer prevent disease from organisms that usually don’t cause illness in people with normal immune function. In healthy individuals, these organisms are kept in check, but when immunity is compromised—due to HIV, chemotherapy, organ transplantation with immunosuppressants, or other conditions—they can take hold and cause infection. This description best fits opportunistic infections. The other statements aren’t characteristic: infections that occur only after organ transplant are too narrow to define the concept, infections caused by any virus during pregnancy aren’t about immune weakness in the same way, and infections that always cause severe illness in healthy people contradict the idea that these infections exploit a weakened immune system.