What is opportunistic pathogen? are they bacteria, viruses, or what?

OK Microbes ...: A person with a healthy immune system will have certain bacteria, fungi, viruses, etc that colonize the body without causing disease. However, given the "opportunity" (i.e. Immunodeficiency, barrier compromise), these same microbes can cause disease. For example, staph bacteria found on human skin doesn't cause problems unless a skin cut allows the bacteria to get in and cause an infection.