DISEASES CAUSED BY UNKNOWN PARASITES 313 
caused by microérganisms too small to be seen by our microscopes. 
That they are caused by living agents of extremely minute size 
is shown by two series of facts: (1) Material may be obtained 
from animals suffering from the disease which will produce the 
disease in others, but its power is destroyed by the same dis- 
infectants as those used to kill bacteria. (2) The infectious agent 
will pass through porcelain filters, whose pores are too small to 
permit even the smallest bacteria to pass, while it will not pass 
through some of the very fine porcelain filters with pores still 
smaller, but large enough to allow liquid to pass through them. 
There are other reasons for the conclusion, but they cannot be 
given here. Although these organisms have never been seen, 
quite a little is known of their general nature. The animal dis- 
eases produced by invisible organisms are the following. 
Foot-and-mouth Disease.—This disease, manifesting itself 
chiefly in the mouth and feet of cattle, varies, much in its severity. 
Although not often causing death, it does result in great financial 
losses to dairymen. It is readily transferred to other animals, 
most kinds being susceptible to it. It occurs rarely in man, being 
transported through the milk of diseased animals, but it is not a 
very serious matter, being a mild infection only. For many years 
it has caused heavy losses in the cattle-raising communities of 
Europe. It has not been common in the United States, though a 
few cases have occurred at intervals, and there have been a few 
rather severe epidemics within the last ten years. These epi- 
demics have been vigorously handled by the agricultural depart- 
ment and have been speedily stamped out. Itis hoped that by the 
vigorous measures taken in killing all cattle attacked, the disease 
may be prevented from gaining a foothold in the country, and that 
our dairymen may thus be protected from the troubles and losses 
experienced elsewhere. Hitherto the efforts have been successful. 
No other remedy is known save that of isolation or slaughter. 
Rinderpest. Cattle Plague.—This is a very serious disease, 
originally found in Asia, but for centuries periodically invading 
