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..