A description usually applied to infection resulting from an organism that does not normally cause disease in a healthy individual. The body’s defense mechanism can usually combat these organisms, but if it is impaired as happens in AIDS/HIV or other immune deficiencies opportunistic infection may develop. Some viral and fungal infections behave in this way. Antimicrobial treatment is often effective, even though the weakness in the body’s defense mechanism cannot be rectified.
Manifesting its pathogenic potential selectively, the onset of the disease occurs solely under particular circumstances, notably when an individual is already afflicted by an existing ailment or harbors a compromised immune system. It is within these specific conditions that the disease can take hold, exploiting the vulnerability presented by the weakened state of health or diminished immune defenses.