We think of ourselves as independent creatures, acting and moving separately and only occasionally interacting with other humans, animals and the environment. But this is a misunderstanding.  We are protected by several surfaces which are in constant and intimate contact with outside influences. Let’s take them one at a time:

The skin This protects most of the body by being flexible and capable of healing after a minor injury. It also produces secretions which are waterproof and assist in the healing process. The skin responds to and is the first line of protection against attacks by insects, trauma, chemicals and radiation. There are friendly bacteria living on the skin which may assist in preventing infections.

Other exterior surfaces (and their protective processes), such as the corneas at the front of the eyes (tears), the ear canal (wax) and interior of the nose (secretions), are all liable to attack by outside influences and have protective mechanisms in place.

The mouth is especially at risk, is quick to sense potential damage from sharp objects and damaging chemicals, and its lining heals rapidly. Once again there is a normal colony of bacteria in the mouth which may have a protective role.

The lungs are in constant contact with the outside world. Air, first filtered through the nose and over the sticky surfaces of the trachea, rushes into the innermost crevices of the lungs carrying with it a wide variety of wind borne dust and smoke particles, pollen grains and spores of fungi. Remarkably, most are trapped and removed by the mucous escalator which carries them up to be swallowed and harmlessly passed in the faeces. There is evidence, however,  that very small particles, such as those produced by the burning of diesel fuel, are not rejected and can cause long term damage. Viruses, also, cannot be easily eliminated, hence the ease with which flu and other respiratory ailments can be spread.

The colon lining is another surface which is constantly in touch with non-human life. The colonic bacteria have evolved to assist us with the breakdown of otherwise indigestible substances, and they are in intimate contact with the lining of the colon. The colon lining may assist with the nutrition of these friendly bacteria and it absorbs the substances which the bacteria produce. We have yet to fully understand the use to which our bodies make of these bacterial substances. It is also likely that a healthy colony of our friendly bacteria prevent us being attacked by any dangerous bacteria which make their way to the colon.

We have evolved to cope continuously with and mostly resist many external dangers. The lesson is to know how this occurs and not interfere with or damage the protective processes. The use, for example, of caustic soaps and disinfectants can remove the protective secretions, and some of the friendly bacteria, from the the skin and leave it more open to infection. And the integrity of the lining of the colon, and ultimately our health, depends on keeping the friendly bacteria therein properly fed and active. We cannot avoid being in contact with and influenced by our environment, but need a better understanding of how the human animal has evolved to cope with the daily intrusions on our integrity.