HAMILTON SCIENTIFIC ASSOCIATION. 39 



best at body temperature — 35 to 37° C. Heat at 55 to 70° C. de- 

 stroys, while cold merely inhibits growth. Moisture is absolutely 

 necessary for most forms and desiccation is almost uniformly fatal. 

 Some require oxygen. Others die unless oxygen is excluded from 

 their feeding grounds. Light, especially direct sunlight, is one of 

 our most important germicides. In the laboratory we are able to 

 grow bacteria on such media as beef broth, gelatin, potato, litmus, 

 agar, glucose bouillon and blood serum. In growing, the bacteria 

 give rise to various products, some of which are poisonous to the 

 human system, and on this account are called toxins, while the 

 parasites producing them are called pathogenic bacteria. These 

 pathogenic forms are the cause of such infectious diseases as 

 typhoid, diphtheria, pneumonia, cholera, tuberculosis, menin- 

 gitis, etc. 



Germs are found everywhere — in the air we breathe, in the 

 water and milk we drink, in the food we eat, in our homes, on the 

 streets, in the cars. They attach themselves to our clothing, 

 books, papers, etc. We carry them around on our hands, in our 

 hair, in our mouths. They come through the mail in our letters, 

 they cling to dollar bills even closer than we are apt to ourselves. 

 They ride on our bread tickets and milk bottles. They lurk in 

 cellars and on street corners. They play around back yards and 

 public alleys. They float up through the manholes from our 

 sewers. They freeze up in our ice and occupy our refrigerators in 

 the summer. In short, they are everywhere, ready to seize for 

 prey any living organism that offers them a foothold. Truly, we 

 are as *' sheep in the midst of wolves." 



But this dark picture has a brighter side and this brings me to 

 my subject proper. Immunity. 



By immunity, we understand that phenomenon in which an 

 individual or a whole class of animals exhibits such resistance to 

 an infection as to withstand it, while under the same conditions 

 another individual or another class of animals may yield to it. 



Immunity may be natural or acquired. Many diseases, which 

 attack man, cannot be inoculated into the lower animals, e.g., scar- 

 let fever, measles. Neither is man susceptible to many of the 



