THE REL.\TI<>\ OF BACTERIA TO DISEASE 161 



lesion, the diphtheria bacillus but a very minute infected area, but the 

 streptococcus may give rise to a severe cellulitis or fatal septicaemia. 

 Now place the same bacteria on an abrasion in the throat. The 

 typhoid bacillus is again harmless; the diphtheria bacillus produces 

 inflammation, a pseudomembrane, and toxaemia, and the streptococcus 

 causes an exudate, an abscess, or a septicaemia. Finally, introduce the 

 same bacteria into the intestines, and now it is the typhoid bacillus 

 which produces its characteristic lesions, while the streptococcus and 

 diphtheria bacillus are usually innocuous. 



If we tried in this way all the parasitic bacteria we would find that 

 certain varieties are capable of developing and thereby exciting dis- 

 ease only on the mucous membrane of the throat, others of the intes- 

 tine, others of the urethra; some develop only in the connective tissues 

 or in the blood, while others, again, under favorable conditions, seem 

 able to grow in or upon most regions of the body. 



The Dissemination of Disease. The spread of infection is influenced 

 by: 1. The number of species of animals subject to infection. 



Many human infectious diseases do not occur in animals, and many 

 animal infections are not found in man. Thus, so far as we know, 

 gonorrhoea, syphilis, measles, smallpox, typhoid fever, etc., do not 

 occur in animals under ordinary conditions; while tuberculosis, anthrax, 

 glanders, hydrophobia, and some other diseases are common to both 

 man and animals. 



2. The quantity of the infectious material and the manner in which 

 it is thrown off from the body. 



In diphtheria, typhoid fever, cholera, pulmonary tuberculosis, septic 

 endometritis, influenza, and gonorrhoea enormous numbers of infec- 

 tious bacteria are cast off through the discharges from the mouth, 

 intestines, and genitourinary secretions, causing great danger of infec- 

 tion. On the other hand, in tuberculous peritonitis, relapsing fever, 

 septic pleurisy and endocarditis, gonorrhreal rheumatism, and the like, 

 there is little or no danger of infecting others, as few or no bacteria are 

 cast off. 



3. The resistance of the infectious bacteria to the deleterious effects 

 of drying, light, heat, etc. 



In this case the presence or absence of spores is of the greatest impor- 

 tance. The spore-bearing bacilli, such as tetanus, anthrax, etc., being 

 able to withstand destruction for a long time, retain their power of 

 producing infection for months or even years after elimination from 

 the body. The bacteria which form no spores show great variation in 

 their resistance to outside influences. Some of these, such as the influ- 

 enza bacilli and the gonococci, the virus of syphilis and hydrophobia, 

 are extremely sensitive; the pneumococci, cholera spirilla, glanders 

 bacilli, etc., are a little hardier; then follow the diphtheria bacilli, 

 and after them the typhoid and tubercle bacilli and the staphylo- 

 cocci. 



4. The ability or the lack of ability to grow outside of the infected 

 tissues. 



