412 A YEAR IN SCIENCE 



his arm or injure some internal organ. Parasites, such 

 as the tapeworm, or trichina, may infest organs and 

 thus interfere with their working. 



In many diseases the causes are not so evident, but 

 we .now know that they are the result of the action of 

 very small plants and animals. They are commonly 

 called bacteria, germs, micro-organisms, or microbes. 

 These small organisms either act directly upon the tis- 

 sues, or they produce poisons, called toxins, which 

 destroy the tissues or in some other way prevent their 

 .normal activity. 



The more common plant germs, we have already 

 learned, are bacteria. Diseases of which we know they 

 are the cause are diphtheria, measles, tuberculosis, 

 pneumonia, mumps, scarlet fever, tetanus (lock jaw), 

 cholera, plague, and spinal meningitis. Small one- 

 celled animals, protozoa, are likewise responsible for 

 some diseases, the best known of which are malaria, 

 smallpox, and yellow fever. All diseases which are 

 caused by germs are "catching" or contagious. 



How germs enter the body. Germs are found every- 

 where about us, in the air, in the water, in our food, 

 and on our clothing. Fortunately, not all of them 

 succeed in entering the body, and even if some of them 

 do get in they do not always find conditions there 

 favorable to growth. 



Germs must enter the body through the digestive 

 tract, the respiratory organs, or the skin. Those which 

 enter the digestive tract are taken in with our food 



