Discrimination between nosocomial and community infections is important for investigation and prevention. Nosocomial and hospital-acquired infections require appropriate hospital control measures to avert additional cases. Nosocomial infections (NI) occur during hospitalization or are caused by microorganisms acquired during hospital stay. Such infections should not be evident when patients are admitted to the hospital. Furthermore, the definition of NI is based on epidemiological criteria, such as the time lapse between admission and onset, or microbiological criteria. This definition might be difficult to apply to invasive aspergillosis (IA) which often afflicts patients with severe immunosuppression or transplantation. Identification of the source may be difficult which could arise outside or inside the hospital. Another significant issue is the lack of valid and reproducible data on the incubation period. The incubation duration of IA is influenced by different individual or environmental determinants, including the severity of immunosuppression and air quality. The criteria of causality are also a means of discussing the contribution of hospital vs. community determinants of IA. The definition of nosocomial IA remains difficult. A better understanding of early events related to IA onset will help to prevent this disease for which the prognosis remains negative.