310 BIOLOGY AND HUMAN LIFE 



of bacteria are found in various industrial applications and in 

 the disposal of sewage (see page 431). 



239. Communicable diseases. Before bacteria can cause dis- 

 ease they must enter the body of the host. Ordinarily they can- 

 not get through the skin. The infection, or entrance into the 

 body, takes place (i) through a cut or break in the skin or 

 (2) through one of the regular openings to the interior of the 

 body, as the mouth or nose. 



Since the time of Pasteur many physicians and biologists 

 have succeeded in finding the particular species of parasites con- 

 nected with some of the most important diseases of man, such 

 as tuberculosis, diphtheria, typhoid fever, tetanus, cerebrospinal 

 meningitis, and influenza. The methods worked out have been 

 used successfully in preventing the spread of many other dis- 

 eases, although the specific organisms that cause the diseases 

 are not known, for in addition to discovering that a specific 

 disease is caused only by a particular species of parasite (for 

 example, typhoid fever is caused only by the typhoid bacillus, 

 and so on), we have found (i) that the parasites leave the host 

 in special ways; (2) that they are commonly transferred to 

 other hosts in special ways ; and (3) that they enter the bodies 

 of new hosts in special ways. 



The table on the opposite page tells how the germs of a num- 

 ber of common diseases are thrown off from their host, how they 

 are carried about, and how they enter the bodies of new hosts. 



There are three different points at which we might plan to 

 prevent the spread of the diseases mentioned : 



1. At the point of leaving the first host. This would mean, 

 if not preventing the bacteria from getting out, at least protect- 

 ing discharges from the body, and objects that come in contact 

 with the host or with these discharges, and perhaps killing the 

 parasites at this point. 



2. In the course of transmission. This has to do with the 

 objects that come in contact with the sick person and with the 

 discharges. The air which separates people cannot very well 

 be eliminated, but, like the other objects and materials that 



