A type of infection that is regarded as a sign of a pathological susceptibility to infection. There are five general subtypes. (i) Opportunistic infection, meaning infection by a pathogen that is not normally able to cause infection in a healthy host (e.g., pneumonia by Pneumocystis jirovecii or CMV); (ii) Unusual location (focus) of an infection (e.g., an aspergillus brain abscess); (iii) a protracted course or lack of adequate response to treatment (e.g., chronic rhinosinusitis); (iv) Unusual severity or intensity of an infection; and (v) unusual recurrence of infections.