In one aspect of the invention, there is provided a method for early detection of subacute, potentially catastrophic illness in an infant. The method comprises: (a) monitoring heart rate variability in the infant; and (b) identifying at least one characteristic abnormality in the heart rate variability that is associated with the illness. This method can be used 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 for early detection of subacute, potentially catastrophic illness in an infant, which comprises: (a) monitoring the patient's RR intervals; (b) generating a normalized data set of the RR intervals; (c) calculating one or more of (i) moments of the data set selected from the third and higher moments and (ii) percentile values of the data set; and (d) identifying an abnormal heart rate variability associated with the illness based on one or more of the moments and the percentile values.