Final answer:
An ordinarily harmless bacterium that becomes pathogenic under certain conditions, such as a compromised immune system, is known as an opportunistic pathogen.
Step-by-step explanation:
An ordinarily harmless bacterium that may cause disease if a patient has a weakened immune system or is under unusual stress is called a opportunistic pathogen. This type of pathogen contrasts with a primary pathogen, which can cause disease in a host regardless of the host's resident microbiota or immune system. Opportunistic pathogens can only cause disease when the host's defenses are compromised, such as by a breach in body's protective barriers, immune system weaknesses, or alterations in normal microbiota.
Conditions that make individuals vulnerable to these opportunistic infections include being very young or elderly, being pregnant, undergoing chemotherapy, having immunodeficiencies like AIDS, recovery from surgery, or having severe wounds or burns.