ANSD level; HFMD; autonomic nervous system dysfunction; cardiology; classifying ANSD levels; difficult problem; diseases; electrocardiogram; electrocardiography; enormous healthcare resources; feature extraction; frequency domains; health care; high mortality rate; infectious disease; learning (artificial intelligence); low-cost wearable sensors; medical computing; medical signal processing; middle-income countries; neurophysiology; patient care; patient diagnosis; patient treatment; photoplethysmogram waveforms; physiological patient data; proof-of-principle; resource-demanding; serious infectious diseases; severity detection tool; standard heart rate variability analysis; support vector machine; support vector machines; tetanus patients; young children