opportunistic (op-er-tew-nis-tik) adj. denoting a disease that occurs when the patient's immune system is impaired by, for example, an infection, another disease, or drugs. The infecting organism rarely causes the disease in healthy persons. Opportunistic infections, such as Pneumocystis pneumonia and those caused by Mycobacterium (see MAI complex), are common in patients with AIDS.

opportunistic Describing a species that can quickly exploit new resources as they arise, for example by rapidly colonizing a new environment. Such species characteristically exhibit r selection.