How are opportunistic pathogens defined?

Opportunistic pathogens are defined as microorganisms that typically do not cause disease in healthy individuals but can lead to infections in individuals with weakened immune systems or other health conditions. This group includes a wide range of bacteria, viruses, fungi, and parasites that usually coexist harmlessly with their host. However, when the host's defenses are compromised—due to factors like immunosuppression, chronic illness, or invasive medical procedures—these organisms can take advantage of the altered environment and cause opportunistic infections.

This definition highlights the importance of a strong immune system in preventing infections from these pathogens under normal circumstances. In the case of healthy individuals, these microorganisms might be part of the normal flora and play beneficial roles; however, their opportunistic nature becomes evident when the host's immunity is impaired.