Considered in its biological relations, infection is the adaptation of an organism to the environment which the body of the host offers.  It is rather singular that variations in organisms represented by such adaptation do not more frequently arise, in which case new diseases would frequently occur.  It cannot be denied that new diseases appear, but there is no certain evidence that they do, and there is equally no evidence that diseases disappear.  From the meagre descriptions of diseases, usually of the epidemic type, which have come down to us from the past, it is difficult to recognize many of the diseases described.  The single diseases are recognized by comparing the causes, the lesions and the symptoms with those of other diseases, and new diseases are constantly being separated off from other diseases having more or less common features.  Many new diseases have been recognized and named, but it is always more than probable that previously they were confounded with other diseases.  Smallpox is such a characteristic disease that one would think it would have been recognized as an entity from the beginning, but although the description of some of the epidemics in remote times conform more or less to the disease as we know it, the first accurate description is in the eighth century by the Arabian physician Rhazes.  Cerebro-spinal meningitis was not recognized as a separate disease until 1803, diphtheria not until 1826, and the separation between typhoid and typhus fever was not made before 1840.  Nor is it sure that any diseases have disappeared, although there seems to have been a change in the character of many.  It is difficult to reconcile leprosy as it appears now with the universal horror felt towards it, due to the persistence of the old traditions.  It is possible, however, that the disease has not changed its character, but that such diseases as smallpox, syphilis, and certain forms of tuberculosis were formerly confounded with leprosy, thus giving a false idea of its prevalence.

In certain cases the adaptation of the organism is for a narrow environment; for example, the parasitism may extend to a simple species only, in others the adaptation may extend to a number of genera.  In certain cases the adaptation is mutual, extending to both parasite and host and resulting in symbiosis, and this condition may be advantageous for both.  Certain of the protozoa harbor within them cells of algae utilizing to their own advantage the green chlorophil of the algae in obtaining energy from sunlight and in turn giving sustenance to the algae.  Although the algae are useful guests, when they become too numerous the protozoan devours them.  It is evident that symbiosis is the most favorable condition for the existence of the parasite, and an injurious action exerted by the parasite on the host unfavorable.  The death of the host is an unfortunate incident from the parasite’s point of view in that it is deprived of habitation and food supply, being placed in the same unfortunate situation as may befall a social parasite by the death of his host.