Any organism that is able to induce disease only if the host's immune or other defenses are compromised.
A microorganism that does not cause disease in a healthy host, but only in individuals whose normal defense mechanisms have been compromised (e.g., burn patients, recipients of organ transplants receiving immunsuppresants, individuals with AIDS, pregant women) (Lecture: Overview of Bacterial Pathogenesis, 1/22/02)
An organism that only causes disease when host resistance is abnormally low.