What defines an opportunistic infection?

An opportunistic infection is defined as an infection that occurs when the immune system is weakened. This condition allows normally non-threatening organisms, which might typically be kept in check by a healthy immune system, to exploit the opportunity presented by a compromised immune response. Individuals with weakened immune systems, such as those with HIV/AIDS, cancer patients undergoing chemotherapy, or organ transplant recipients, are particularly susceptible to these types of infections.

In contrast, an infection caused by a known pathogen typically refers to infections that can occur in healthy individuals, where the pathogen is recognized and can provoke a disease regardless of the immune status. Similarly, stating that an infection arises when the immune system is strong does not align with the concept of opportunistic infections, as these infections specifically thrive in situations where immunity is lacking. Moreover, infections can be caused by a wide variety of organisms, including bacteria, fungi, and parasites, not just viruses, which further distinguishes opportunistic infections from exclusive viral infections.