What type of infection occurs when the immune system cannot defend itself against common pathogens?

An opportunistic infection occurs when the immune system is weakened or compromised, making an individual more susceptible to infections from pathogens that typically wouldn't cause illness in healthy individuals. These pathogens can include bacteria, fungi, and viruses that are considered "opportunistic" because they take advantage of the body's lowered defenses.

In healthy individuals, the immune system effectively combats these common pathogens, but when a person's immune response is diminished—due to factors such as immunodeficiency disorders, certain medications, or chronic diseases—these normally harmless organisms can lead to serious infections. This is why opportunistic infections are particularly prevalent in individuals with conditions like HIV/AIDS, cancer, or those undergoing immunosuppressive therapy.

Acute, chronic, and viral infections refer to different aspects of infection types, where acute infections are typically sudden and severe, chronic infections persist over time, and viral infections are specifically caused by viruses. However, none of these terms specifically relate to the failure of the immune system to protect against common pathogens, which is the defining characteristic of opportunistic infections.