MEANING OF DISEASE 453 
very susceptibility invites attack, and if the infection is 
intensely virulent death may threaten or follow. 
8. Course of disease—Each disease has its own pecul- 
iar characteristics. These are more or less conspicuous 
in each individual case. Some 
diseases develop quickly and 
end quickly. Others run. a 
course of several months or 
even years. The first class is 
acute, the second chronic. In 
both kinds nature always en- 
deavors to effect a cure; and, 
unless other complications 
arise from improper food, in- 4 position d during 
sanitary quarters, bad air or the course of this ailment. 
conditions not conducive to health, recovery in most 
cases will result. 
9. How disease runs.—The course of a disease in a 
general way is known before it makes its attack. Physi- 
cians and veterinarians know when a fever, for instance, 
will begin, how long it will last, when it will be at its 
highest point, and when it will disappear. They know 
these facts even before they begin their treatment. Yet 
no disease invariably runs the same course in all animals. 
The virulence of bacteria has much to do with care in 
treating; mild cases occur usually when the germs are 
weak, and severe cases when the germsare virulent. This 
explains why some attacks of measles or Texas fever or 
hog cholera are more fatal than other attacks in other 
places or at other seasons of the year. 
10. Period of incubation —In the regular course of an 
infectious disease, the period of incubation follows infec- 
tion. During this period, no change in the health of the 
animal is observed. It seems well, acts well, and does 
WHEN COoLic ATTACKS 
