home / medterms medical dictionary a-z list / opportunistic microorganism definition Medical Definition of Opportunistic microorganism Opportunistic microorganism: A bacterium, virus, protozoan or fungus that takes advantage of certain opportunities to cause disease. Those opportunities are called opportunistic conditions. These microorganisms are often ones that can lie dormant in body tissues for many years, such as the human herpes viruses, or that are extremely common but usually cause no symptoms of illness.