Final answer:
An opportunistic infection occurs when normally harmless microorganisms cause disease, typically when a person's immune defenses are weakened or if the microbes enter a part of the body they do not normally inhabit.
Step-by-step explanation:
An opportunistic infection is caused by a microorganism that is normally beneficial or harmless to humans. These microbes, part of the body's normal flora, become pathogenic under certain conditions such as a compromised immune system or if they are introduced to an area of the body that is normally sterile.
For example, E. coli which is typically found in the large intestine can cause a urinary tract infection if it enters the bladder. Similarly, Staphylococcus aureus, including the methicillin-resistant strain MRSA, may be harmless on intact skin but can cause serious infections if they enter other parts of the body. This is particularly problematic during and after surgeries or when the skin's integrity is breached.
Individuals most susceptible to opportunistic pathogens include those with weakened immune systems, such as patients undergoing chemotherapy, people with HIV/AIDS, the elderly, and those recovering from surgery.