368 SCIENCE OF HOME AND COMMUNITY 



Conditions of growth. The most important conditions 

 affecting the growth of bacteria are temperature and moisture. 

 For the majority of bacteria the most favorable temperature 

 is between 60 and 100 degrees. The effect of lowering the 

 temperature is to lessen their activity and they become 

 dormant near the freezing point. At this point most 

 kinds are killed,' but some can withstand freezing for several 

 weeks or months. Such are the bacteria that cause typhoid 

 fever, which may be frozen in the ice for two months during 

 the winter and when the ice melts some may renew their 

 activities. All bacteria, however, are killed by boiling, and, 

 if the heating is continued for an hour, they may be destroyed 

 at a temperature of 160 degrees. There is one form in which 

 bacteria exist, known as spores, the vitality of which is not 

 destroyed by bringing it to the boiling point. Under certain 

 conditions some bacteria may stop their activities and form 

 themselves into a spherical mass surrounded by a thick mem- 

 brane. These spores are dormant but are able to withstand 

 adverse conditions, such as excessive drying and heat which 

 would kill bacteria in their ordinary forms. But while 

 spores are not destroyed by boiling a short time, if the boil- 

 ing is continued for several hours, the spores will be killed. 



Two other conditions favorable to the growth of bacteria 

 are dampness and darkness. They cannot withstand dry- 

 ing and the action of direct sunlight, hence the need of having 

 our houses well lighted and our windows unobstructed. 



Relation to disease. The bacteria in their action within the 

 body produce poisons, some of which are called toxins, which 

 are absorbed into the blood and carried through the body, 

 producing the ill effects characteristic of each disease. All of 

 these diseases may be transmitted from one person to another ; 

 some, however, are more highly contagious than others. It is 

 very important that everybody should know something about 

 these diseases ; how they are caused, how they may be carried, 

 and how they may be avoided. Then every one will under- 



