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.