8 Parasitic Bacteria and their Relation to Saprophytes,  [Jan. 
I have already stated that there are many entozoa, inhabiting 
the tissues of their host, which do but little harm, and which may 
measure their parasitic existence by years, while a few, such as 
Trichina spiralis, are now and then fatal. Corresponding with _ 
these gradations in destructive effect there are similar gradations 
of virulence among bacteria. Some produce only local disturb- 
ances ; they are speedily destroyed and eliminated. Among these 
are the microbes causing suppuration. Others destroy organs 
and tissues very gradually, and are indirectly fatal by exhausting 
the vital energies or breaking down some organ necessary func- 
tionally to the processes of life. Among these may be mentioned 
more particularly the tubercle bacillus. Still others may cause 
death from within a few hours to weeks after their invasion. 
These include the microbes of septiczemia, cholera, typhoid fever. 
In general, however, the tendency of bacterial parasites is emi- 
nently destructive. The chemical poisons formed during their 
growth irritate and finally destroy the animal cell. If we pass 
from a consideration of the biology of these micro-organisms to 
the diseases of which they are the cause, a broad field of inter- 
esting facts lies before us, as instructive and suggestive ‘to the 
biologist and the student of nature as to the pathologist and the 
practical physician. I can, however, merely glean a few facts 
which may serve to illustrate the relation of epidemics_to the life- 
history of bacteria. 
There is a certain group of diseases called miasmatic, because 
the poison seems to come from the air and the soil. With the 
light shed upon this subject in recent years, the micro-organisms, 
presumably the cause, live in the soil as their natural habitat. 
This class would include all strictly endemic diseases, since they 
cannot be carried at will to localities free from them. The cause, 
residing in the soil, must have certain conditions necessary to its 
life, and unless these are found in new localities the disease will 
not take root. Though malaria is reaching out into new terri- 
tory, we have never yet heard of a quarantine against its progress. 
Another group includes maladies only transmitted from one 
subject to another. They are strictly contagious diseases, corre- 
sponding to the strictly parasitic bacteria, which cannot multiply 
outside of the animal body. 
A third group, intermediate between these extremes, possesses, 
i coe in a = bed characteristics of both. The micro-organisms may 
