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.
Is bacteroides fragilis considered a regular bacteria that can get anyone, or an opportunistic pathogen?
Somewhere in between. B fragilis is somewhat rare among gut flora- namely the bacteria that usually live in the gut, often less than 1% of the total types of bacteria. It is not opportunistic in the sense that it needs HIV or chemotherapy to do its work, but does need to take advantage of another injury or infection, such as an anal burn, an abcess or an intestinal rupture to cause serious infections.