What defines an opportunistic pathogen?

An opportunistic pathogen is defined as a microorganism that typically causes disease when the host's immune defenses are compromised or weakened. These pathogens usually do not cause disease in healthy individuals with a robust immune system; instead, they exploit situations where the immune response is insufficient, such as in immunocompromised patients, those with chronic diseases, or individuals undergoing treatments that impair their immune function.

For instance, pathogens such as certain strains of Staphylococcus or Candida can be found as part of the normal flora in healthy individuals but can lead to serious infections in those with weakened defenses. This characteristic underscores the context-dependent nature of these pathogens in relation to the host's immune status. Supporting this, while some microorganisms may be beneficial under normal circumstances, opportunistic pathogens can lead to infections under specific conditions, which distinguishes them distinctly from purely pathogenic microbes that consistently cause disease in any host.