RESISTANCE AGAINST MICROORGANISMS. 253 
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 live is small. 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 substances, 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 individual, some members 
of a species having the resisting power highly developed, while 
others yield easily to invasion. This we speak of as individual 
resistance. The resistance varies also with the vigor of the germs. 
Some epidemics of measles, for example, are mild and some severe, 
and a person's resistance against an attack is partly dependent upon 
the vigor of the germs. 
Now, this resisting power is clearly located in the living cells of 
