TO SOME DISEASES OE STOCK 
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and the germ which it conveys belongs to the lowest form of 
animal life, the last zoological phylum it is necessary to mention 
— the family Protozoa. 
When one speaks of disease-producing germs, one refers 
to microbes of three known categories : — 
1. Bacteria, which include the causes of typhoid, tuber- 
culosis, anthrax, and many others, all of which are classed as 
vegetable and represent the lowest form of plant life. Usually 
they do not possess any complex life history, but multiply by 
simple fission, and, in some cases only, form a resistant spore, 
by means of which adverse conditions, fatal to the vegetable 
cell itself, can be resisted and life maintained. 
2. Protozoa, in the lower forms of which multiplication 
is by simple fission, but in the higher we find the very com- 
plicated sexual and asexual cycles analogous to that indicated 
for the parasite of malaria. Many disease-producing forms 
of Protozoa — such as malaria, East Coast fever, and tick fever 
of animals — require the intermediation of an insect host in 
which the sexual cycle is completed ; others — such as the 
amoeba of dysentery and the coccidia, which cause gastro- 
enteritis in cattle — do not require an intermediary host ; and 
in the case of coccidia the later stages of the sexual cycle are 
undergone on the ground. 
3. Thirdly, there exists a group of germs — if they can be 
so called — so small as to be invisible to the strongest microscope. 
Their presence in cases of disease can be proved by experi- 
ment, and it can further be shown that these invisible organisms 
are of different sizes, because some will readily pass through 
filter candles of very fine porosity, while others, though capable 
of passing through coarser candles, will be withheld by these. 
The ordinary filter-candle used for domestic water-supply will 
permit the passage of all germs of this category, yet they retain 
even very small bacteria. 
In view of their minute size and invisibility, it is readily 
understandable that discussion has ensued as to the nature 
of these so-called ultra visible viruses ; but since with some of 
them it has been proved that development takes place within 
an insect host, which is the natural transmitting agent, the 
weight of opinion is in favour of considering them as a special 
