By means of these substances the bacteria produce such changes in their environment within the body that this becomes adapted to their parasitic existence.  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.  Such capsule formation is a very common thing in the pathogenic organisms, and they are found only when these are growing in the body and do not appear in cultures (Fig. 17-c).

It is evident that just as the parasite has his weapons of offence and defence so has the host, otherwise there would be no recovery from infectious diseases.  Although many of the infectious diseases have a high mortality, which in rare instances reaches one hundred per cent, the majority do recover.  In certain cases the recovery is attended by immunity, the individual being protected to a greater or less degree from a recurrence of the same disease.  The immunity is never absolute; it may last for a number of years only, and usually, if the disease be again acquired, the second attack is milder than the primary.  Probably the most enduring immunity is in smallpox, although cases are known of two and even three attacks; the immunity is high in scarlet fever, measles, mumps and typhoid fever.  The immunity from diphtheria is short, and in pneumonia, although there must be a temporary immunity, future susceptibility to the disease is probably increased.  In certain cases the immunity is only local; the focus of disease heals because the tissue there has evolved means of protection from the parasite, but if any other part of the body be infected, the disease pursues the usual course.  A boil, for example, is frequently followed by the appearance of similar boils in the vicinity due to the infection of the skin by the micrococci from the first boil, which by dressings, etc., have become spread over the surface.