Although entropy as a thermodynamically defined measure of order only applies to strictly isolated systems, the concept that nature tends to spontaneously revert to disorder helps to explain threats to our health - both as individuals, and as societies.

The complexities of our structures as living organisms, and of our societies as interactive communities, are both apparent contradictions of the principle of entropy as time's arrow. The processes that maintain these contradictions are central to our health and survival. This is illustrated by our own studies of the protein that controls the natural anticoagulant state of blood.

This protein exists in a metastable form, which readily undergoes a spontaneous and irreversible change to an inactive form. Even at normal body temperature, a million million molecules per second undergo this inactivating transition, with any increase in this rate being liable to tip the anticoagulant balance of the blood - with the consequent onset of thrombosis. The balance is only maintained by a constant replacement of the anticoagulant protein, but not all the proteins in the body are capable of similar regeneration. The summation of such spontaneous molecular changes, over many years, explains aging, and the inevitable senescence that will always limit the length of life.

The highly ordered molecular interactions that are necessary for life are also vulnerable to disruption by external factors, such as viruses, or self-propagating prion proteins. To counter this, humans have evolved a series of barriers, that protect our biological integrity. But these barriers can still be overwhelmed, to give infection. The health of our communities, and the survival of our species, is then dependent upon the limitation of these infections to the individual.

For this reason, our societies have developed mores and traditions, to restrict the intimate contacts that favour the spread of infection. Warnings of the danger to our societies, when these safeguards break down, has come with the recent outbreaks of blood-borne infection such as HIV - where changes in mores have been compounded by well-intentioned therapeutic innovations, such as the transfusion of pooled blood products.