What type of infection is typically caused by opportunistic pathogens in individuals with poorly functioning immune systems, such as those with AIDS?

Opportunistic infections are caused by pathogens that usually do not cause disease in a healthy immune system but can lead to infections in individuals with compromised immune systems. In the context of individuals with poorly functioning immune systems, such as those with AIDS, these infections arise due to the body's inability to effectively fight off these normally harmless organisms.

Patients with AIDS have significantly reduced levels of CD4+ T cells, which play a crucial role in the immune response. This immunosuppression allows opportunistic pathogens—including certain bacteria, viruses, fungi, and parasites—to exploit the weakened defenses and lead to severe infections. Conditions like Pneumocystis pneumonia, candidiasis, and cytomegalovirus infections commonly occur in this population, illustrating how opportunistic pathogens take advantage of the immune system's vulnerabilities.

In contrast, systemic infections typically spread throughout the body and may involve multiple organ systems, while localized infections are confined to a specific area. Nosocomial infections, on the other hand, are acquired in a healthcare setting, which can affect any patient but are not specifically linked to immunocompromised patients. Thus, the defining feature of opportunistic infections is their propensity to cause illness primarily in those with weakened immune responses.