Abstract: In one aspect of the invention, there is provided a method and apparatus for early detection of subacute, potentially catastrophic infectious illness in a premature newborn infant. The method comprises: (a) continuously monitoring heart rate variability in the premature newborn infant; and (b) identifying at least one characteristic abnormality in the heart rate variability that is associated with the illness. This method can be use to diagnose illnesses such as, but not limited to, sepsis, necrotizing enterocolitis, pneumonia and meningitis. In another aspect of the present invention, there is provided a method and apparatus for early detection of subacute, potentially catastrophic infectious illness in a patient.