454 Our Surroundings 



known to be catching, the germs have not yet been discovered. 

 Nevertheless, it is believed that they are of germ origin as they 

 have characteristics similar to those diseases that have been clearly 

 proved to be germ diseases. 



Microorganisms. Germs of disease are always micro- 

 organisms, that is, organisms so minute that they can be seen only 

 by the aid of the compound microscope. Many of them are bac- 

 teria. Diseases such as tuberculosis, typhoid fever, diphtheria and 

 pneumonia are caused by bacteria and are among the most dreaded 

 that afflict mankind. 



Bacteria, however, are not the only forms of microscopic life 

 that cause disease. Single-celled animals called protozoa may also 

 cause them. Malaria, yellow fever, rabies or hydrophobia, sleep- 

 ing sickness and some kinds of dysentery are caused by proto- 

 zoa. 



Protozoa are the cause of disease in animals as well as in 

 man. They sometimes gain a foothold in the tissues of insects, 

 birds, fishes, cattle and other animals, with disastrous results. 

 Horses and cattle in Africa are killed in great numbers by a para- 

 site transferred to their bodies by the tsetse fly. In America many 

 cattle die from Texas cattle fever, or "tick," a disease that is 

 believed to be caused by a protozoan. 



Manners of Infection. In order to control the spread of 

 germ diseases it is necessary to know the routes by which the 

 living germs of each disease leave the body of their victim, the 

 means by which they are transmitted to other persons, and the 

 avenues by which they gain entrance. 



According to the disease, the germs are discharged from the 

 bodies of the sick through the mouth and nose, through the organs 

 of excretion, or through breaks in the skin. 



They are carried to other persons on the hands of those who 

 care for the sick, by flies and other insects, and by pets or by 

 other agencies that have been in contact with the discharges from 

 the bodies of the diseased. 



The paths by which most of these germs enter the body are 

 through the mouth in the food we eat, through the nose in the dust 

 of the air we breathe, and through a wound or break in the skin. 



