The conventional wisdom in medicine holds that disease and aging arise from stress on an otherwise orderly and machinelike system - that the stress decreases order by provoking erratic responses or by upsetting the body's normal periodic rhythms. In the past five years or so we and our colleagues have discovered that the heart and other physiological systems may behave most erratically when they are young and healthy. Counterintuitively, increasingly regular behavior sometimes accompanies aging and disease.
 Irregularity and unpredictability, then, are important features of health. On the other hand, decreased variability and accentuated periodicities are associated with disease. Motivated by these ideas, we and other physiologists have looked for periodic behavior that might indicate developing sickness (especially diseases of the heart). In addition, we have begun to analyze the flexibility and strength of irregular fractal structures and the adaptability and robustness of systems that exhibit apparently chaotic behavior.
Chaos and Fractals in Human Physiology, by Ary L. Goldberger, David R. Rigney and Bruce J. West, Scientific American February 1990, pp.35-41