What is the key feature of an opportunistic infection?

Opportunistic infections are primarily characterized by their occurrence in individuals who have weakened immune systems, such as those with HIV/AIDS, cancer patients undergoing chemotherapy, organ transplant recipients, or individuals on immunosuppressive therapy. In healthy individuals, the immune system typically handles most pathogens effectively, preventing these infections from taking hold.

In contrast, opportunistic pathogens take advantage of the compromised immune defenses, leading to infections that would not occur in a healthy host. This makes recognizing and managing opportunistic infections critical in populations considered immunocompromised, where even typically non-pathogenic organisms can lead to serious disease.

The other choices do not accurately reflect the essence of what defines an opportunistic infection. For example, while some opportunistic infections may require specific antibiotics for effective treatment, this is not a defining characteristic. Additionally, the ability to prevent an opportunistic infection with vaccination depends on the specific pathogen and the individual's immune status, rather than being a generalizable feature of opportunistic infections. Furthermore, these infections are notably uncommon in healthy individuals, which underscores the importance of immune status in their occurrence.