What is an example of a condition that can cause opportunistic pathogens to infect a host?

Opportunistic pathogens are organisms that typically do not cause disease in healthy individuals but can lead to infections in those with weakened immune systems. An immunocompromised status is characterized by a reduced ability of the immune system to fight infections, which can be due to various factors such as certain medical conditions, medications (like chemotherapy), or other health issues.

When the immune defenses are lowered, it opens the door for opportunistic pathogens to invade and establish infections. This is why an immunocompromised individual is more susceptible to infections from pathogens that would normally be harmless to someone with a fully functioning immune system. Thus, this condition serves as a critical risk factor for the establishment of opportunistic infections, making it the correct choice in this scenario.

The other options do not provide the same context for susceptibility to opportunistic infections. A healthy immune system typically protects against these pathogens, stressful life events may temporarily affect health but do not directly lead to susceptibility, and high levels of physical activity are generally associated with better immune function rather than increased risk.