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 algæ utilizing to their own advantage the green chlorophil of
the algæ in obtaining energy from sunlight and in turn giving
sustenance to the algæ. Although the algæ 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.

FOOTNOTE:
[1] Flexner has recently succeeded in isolating and cultivating the
organism of poliomyelitis, but the organism is so small that its
classification is not possible.




CHAPTER VII