314 OTHER GERM DISEASES 
Europe, and recently very rife in Africa. It attacks cattle chiefly, 
and its death rate is very high. Man is immune against it. 
Rabies, or hydrophobia, is also produced by some agent not 
yet surely known. It attacks dogs chiefly, although occasionally 
it is found in horses, cattle, and man. So far as known the only 
source of the disease is the bite of infected animals, and the great 
majority of cases come from the bite of dogs. The name hydro- 
phobia applies to the disease in man only, where a dread of water 
is one of the symptoms. Since this dread of water does not 
appear in dogs suffering from the disease, the name rabies is best. 
applied. 
Other diseases of the same category are: pleuropneumonia, 
a serious disease of cattle; horse sickness, a destructive disease of 
horses in South Africa; bird pest (Vogelpest) a highly infectious 
and fatal disease of chickens; sheep-pox, a disease of sheep in 
the Mediterranean countries. In all of these the exciting agent 
is unknown and is probably too minute to be seen with the 
microscope. 
Hog Cholera.—The hog cholera is a disease which develops 
spontaneously in swine only. It resembles swine plague, but is 
distinct from it, and indeed both diseases may occur in the same 
animal at the same time. The disease sometimes results in very 
serious losses. A herd of swine may be attacked by it in such 
violent form that 90 per cent. of the animals succumb to the 
infection. The disease occurs in an acute form, which runs its 
course with excessive rapidity, producing death in twenty-four 
hours, and in chronic form, which has a slower course, lasting 
from two to four weeks before finally resulting in the death of the 
animal. The cause of the disease is still a matter of dispute. 
Some claim it to be an organism too small to be visible with the 
microscope. Others claim it to be a microscopic animal (Spiro- 
cheie hyos). Until recently a bacillus (B. suipestifer), which 
is generally present in cases of hog cholera, was thought to be the 
causal agent; but this has now been definitely disproved. 
