278 PARASITIC BACTERIA 
sibilities, since all of these methods of treatment lead to abnormal 
conditions and to greater liability to disease. 
The adoption of precautions for preventing the distribution of 
disease germs is doubtless a matter of very great significance; but 
of more significance still is the endeavor so to modify the conditions 
of life as to increase their resisting power against these bacteria. 
In every case, doubtless, the plan adopted will be by the way of 
compromise, and will be such as to give the greatest amount of 
physical vigor consistent with the ends which the farmer has in 
view in his use of the animals. To turn them out into the fields 
with no attempt to produce special types, and with no high feed- 
ing, would doubtless produce a vigorous breed, but it would not 
produce milk. 
ACQUIRED RESISTANCE 
Some species have a perfect resistance to the diseases that other 
species will take, a condition called race immunity. Some indi- 
viduals will resist a disease that others of the same species cannot 
resist, and this is individual immunity. An individual may also 
develop a resistance to a disease which he did not at first possess. 
This is acquired immunity. It has long been known that if a per- 
son has one attack of certain diseases and recovers, he is, for a time 
at least, protected from a second attack of the same disease. It is 
not common, for example, to have scarlet fever twice, and the same 
is true of a number of other diseases. This acquired immunity is, 
however, quite variable. In some cases it is almost a perfect pro- 
tection for life, or at least for many years. With other diseases it 
is weaker, affording only a partial protection and lasting only a few 
months or perhaps only afew weeks. The question of what causes 
this acquired immunity is closely akin to what causes race or indi- 
vidual resistance. Doubtless the two are closely related and are 
probably attributable to the same general cause. For our purpose 
it is only necessary to know that recovery from one of these dis- 
