210 BACTERIA, YEASTS, AND MOLDS 



1 . The causes of these diseases are real tilings and not 

 simply matters of imagination. They can be seen with 

 the microscope ; they feed ; they grow and multiply like 

 larger animals and plants. Contagious diseases are not 

 mere nervous affections that may be banished by forget- 

 ting them and believing in their nonexistence. They are 

 produced by definitely known living beings, and can be 

 avoided only by keeping our bodies free from them. 



2. The causes of the diseases in question are always 

 microscopic, and can never be detected by the naked eye. 

 Material which cannot be seen may therefore be filled 

 with microscopic parasites which are capable of producing 

 fatal diseases. An invisible particle of dust may harbor 

 numbers of deadly germs ready to invade the living body 

 and produce trouble. Since the foes cannot be seen, the 

 battle is a blind and therefore a difficult one. 



3. These agents are alive ; they grow and multiply. 

 Thus it follows that infectious material may rapidly 

 increase in quantity. A particle of dust containing only 

 a few parasitic bacteria may be the starting point of a 

 disease which may spread widely until it shall become an 

 epidemic with its scores of victims. The problem to be 

 dealt with is something like that of fire. The flame of 

 a single match is very slight and may do little injury ; 

 but this same flame may start a conflagration that will 

 burn an entire city. So with the disease bacteria. Each 

 of them, although extremely minute, is capable of develop- 

 ing with wonderful rapidity, and a single one may develop 

 sufficiently in the course of a few days to be scattered 

 far and wide, causing a great epidemic. The extreme 

 minuteness of these foes and their wonderful power of 



