1. A method for early detection of subacute, potentially catastrophic illness in an infant comprising: (a) monitoring the infant's RR intervals; (b) generating a normalized data set of the RR intervals; (c) calculating at least one 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 percentile values.