In symbiosis the bacteria probably undergo changes by which they become adapted to the environment, and in parasitism the environment becomes adapted to them.

In the same way man can change his immediate environment by means of clothing, artificial heating, etc., and adapt it to his needs; or by hardening his body he can adapt it to the environment.

The pathogenic bacterium finds the living tissue hostile, its cells devour him, the tissue fluids destroy him, and by means of the toxines he changes the environment from that of living to dead tissue, or in other ways so alters it that it is no longer hostile.

The parasite has also means of passive defence comparable to the armor of the warrior in the past.

It may form a protective mantle called a capsule around itself, which serves to protect it from the action of the body fluids.