RESISTANCE AGAINST MICROORGANISMS 273 
them are among the most important of the problems before bac- 
teriologists to-day. With these complicated questions, however, 
we are not concerned in this work. 
It is evident enough that some kinds of microdrganisms can 
overcome this resistance, otherwise there would be no parasites. 
These, capable of living and multiplying in the body, may produce 
injury and are the disease germs. Fortunately, the number that 
can thus liveissmall. Many hundred kinds of bacteria have been 
discovered, carefully studied, and described in bacteriological 
literature. We have no idea how many varieties exist in nature, 
but there are certainly hundreds, and perhaps thousands. Of 
these only little more than a score are known that can produce 
disease in man and animals, and a somewhat larger number that 
can produce disease in plants. A few yeasts occasionally produce 
similar troubles. Quite a large number of molds, as parasites, 
give rise to disease in plants, and a very few cause trouble in 
animals. The great host of bacteria and other fungi live upon 
dead matter, and cannot live as parasites. They may spoil foods, 
and destroy wines, beer, butter, cheese, and other valuable sub- 
stances, but they cannot produce disease, since they are not able 
to overcome the resistance offered by the living tissue. 
The body has a resisting power against all kinds of micro- 
Organisms, disease germs as well as the non-parasitic species, 
although, in the case of the former, it is insufficient to prevent 
their invasion. Against the common saprophytes it is perfectly 
efficient; against some parasitic bacteria it is moderately efficient 
and will, in many cases, prevent the development of the disease, 
even after the parasitic bacteria have entered (tuberculosis) ; 
against other bacteria the resisting power is extremely slight 
(anthrax). The resisting power varies with different species of 
animals, some having the power of absolutely resisting certain 
bacteria, when we call them immune. Man is immune against 
hog cholera, while the hog is not. It also varies with the in- 
dividual, some members of a species having the resisting power 
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