Final answer:
Opportunistic pathogens are microorganisms that can cause disease when a person's immune system is compromised. These differ from primary pathogens, which can cause disease without the need for a weakened host. Susceptibility to opportunistic infections varies based on individual health, presence of preexisting conditions, and immune system strength.
Step-by-step explanation:
Opportunistic pathogens are a type of microorganism that may cause disease under certain circumstances, particularly when an individual's defenses are compromised. These organisms differ from primary pathogens, which can cause disease in a host regardless of the host's immune system or resident microbiota. People who are particularly susceptible to opportunistic infections include those with weakened immune systems, such as patients undergoing chemotherapy or those with AIDS, as well as the very young, the elderly, and women who are pregnant.
Opportunistic pathogens can become problematic when a person's protective barriers are breached or when there is a change in the normal microbiota. For example, E. coli, which is typically harmless in the large intestine, can lead to a urinary tract infection if it enters the bladder. Understanding that the potential for disease is not only dependent on the pathogen but also on the health and condition of the host, helps explain why some individuals are more susceptible to certain infections than others.