opportunistic microorganism

opportunistic microorganism A bacterium, virus, or fungus that takes advantage of certain opportunities to cause disease (opportunistic conditions). Opportunistic microorganisms are often ones that can lie dormant in body tissues for many years, such as the human herpesviruses, or that are extremely common but usually cause no symptoms of illness. When the immune system cannot raise an adequate response, these microorganisms are activated, begin to multiply, and soon overwhelm the body’s weakened defenses. See also opportunistic condition; opportunistic infection.