Claude du Bois-Reymond, Berlin,* shows that disease is not a conflict "between parasite and host; it is in truth a kind of imperfect symbiosis." That is, if germs killed us off they themselves would die for want of food. Their existence, then, indicates a delicate balance and we are both able to survive as species, though many individuals perish. The advantage to them is very evident, for without us they would die, but the commensal advantage to us is not so clear. There is an advantage, nevertheless, in being diseased, though we do not know yet what it is. If there were no benefit, the people who promptly killed the invaders would have the advantage in the struggle for existence over those who had their vitality reduced ever so little by an infection, and by the laws of selection, the resistant ones would be the only ones to have offspring. Then, in time, the whole race would resist the germ and the latter would be exterminated. No doubt this comes about eventually in every disease, and new diseases are constantly arising in the passing millenniums, as new species of bacteria are evolved, flourish and then perish from lack of food, but during the process commensalism rules. Death due to parasites, such as typhoid bacilli, may be but the preliminary to an approaching immunity, and some future beneficial commensalism, or, indeed, it may be a disturbance or misplacement of commensal organisms. If that is so, then all disease organisms are commensal even before perfect tolerance is established.
* American Medicine, January 31, 1903.
Adaptation of parasites and their evolution is quite clearly brought out and explained in a paper by Dr. J. G. Adami, of McGill University.* He shows how we evolved along parallel lines with the bacteria. If an ancient Greek would visit us he would promptly die of tuberculosis, pneumonia or influenza. In the last 2,500 years we have developed an immunity which is effective in killing these invading parasites, unless they are too numerous, or we have lost immunity by some other cause of ill health.
When we first found out that we were full of bacteria we were horrified - now we know that those in the mouth dissolve foods lodged between the teeth; those in the skin dissolve dirt in the pores and keep them open; those in the intestines serve some unknown purpose in digestion. Certain worms are found in every fish of certain species, and must do some good to the digestion. Indeed, the infected are so much at an advantage that whenever biologists experiment with an organism by keeping it absolutely sterilized, it invariably dies.
Pasteur is reported to have said, "C'est dans le pouvoir humain de faire disparattre du monde toutes les maladies parasitaires." If there is any truth in the trend of the present thought as to the possible benefit of alleged parasites, we can rest assured that Pasteur's theory is unnatural. To eliminate disease would probably be a disaster, for we are adjusted to the present organisms - indeed, it is known that one disease balances another. Many instances are known where one infection is killed off in another accidentally acquired. Gamier and Sabarneau have reported * experiments which show an antagonism between the germs or poisons of two diseases. It is not at all doubtful that we will find in this direction a use for some of the numerous bacteria in the digestive canal of man - they may be guards ready to kill deadly invaders. Indeed, Metzshnikoff has asserted that if we infect ourselves with lactic acid bacteria found in sour milk, we actually prolong our lives. Charrin, of Paris, has proved that rabbits fed on sterilized food die of starvation, because they naturally depend on some digestive function of the bacteria. We can be too clean - sterilized foods may be fatally bad. Nevertheless cooking of food has had the curious result of causing a loss of immunity to certain organisms, apparently harmless to lower races, who are nests of parasites, and we are now really dependent upon this partial sterilization.
* American Medicine, April 29, 1905. * Archives de Medecine Experimentale.