Again, we have still other parasites which are not thus confined, but which, as soon as they enter the body, produce a general infection, attacking the blood and perhaps nearly all tissues simultaneously.  The most typical example of this sort is anthrax or malignant pustule, a disease fortunately rare in man (Fig. 32).  Here the bacilli multiply in the blood, and very soon a general and fatal infection of the whole body arises, resulting from the abundance of the bacilli everywhere.  Some of the obscure diseases known as blood poisoning appear to be of the same general nature, these diseases resulting from a very general invasion of the whole body by certain pathogenic bacteria.

In general, then, we see that the so-called germ diseases result from the action upon the body of poisons produced by bacterial growth.  Differences in the nature of these poisons produce differences in the character of the disease, and differences in the parasitic powers of the different species of bacteria produce wide differences in the course of the diseases and their relation to external phenomena.

What diseases are due to bacteria?

It is, of course, an extremely important matter to determine to what extent human diseases are caused by bacteria.  It is not easy, nor indeed possible, to do this to-day with accuracy.  It is no easy matter to prove that any particular disease is caused by bacteria.  To do this it is necessary to find some particular bacterium present in all cases of the disease; to find some method of getting it to grow outside the body in culture media; to demonstrate its absence in healthy animals, or healthy human individuals if it be a human disease; and, finally, to reproduce the disease in healthy animals by inoculating them with the bacterium.  All of these steps of proof present difficulties, but especially the last one.  In the study of animals it is comparatively easy to reproduce a disease by inoculation.  But experiments upon man are commonly impossible, and in the case of human diseases it is frequently very difficult or impossible to obtain the final test of the matter.  After finding a specific bacterium associated with a disease, it is usually possible to experiment with it further upon animals only.  But some human diseases do not attack animals, and in the case of diseases that may be given to animals it is frequently uncertain whether the disease produced in the animal by such inoculation is identical with the human disease in question, owing to the difference of symptoms in the different animals.  As a consequence, the proof of the germ nature of different diseases varies all the way from absolute demonstration to mere suspicion.  To give a complete and correct list of the diseases caused by bacteria, or to give a list of the bacteria species pathogenic to man, is therefore at present impossible.