232 BACTERIOLOGY 



only occasional, sporadic disease, but as soon as they are carried 

 beyond this territory, epizootics of high mortality may result. Cli- 

 mate in some cases enters as an important factor, but the most import- 

 ant, perhaps, is the slight elevation in virulence brought about by 

 a more highly resistant host. The most susceptible animals are weeded 

 out, and the rest strengthened by non-fatal attacks. The virulence 

 of the microbe rises slightly to maintain the equilibrium. In passing 

 into a hitherto unmolested territory, the disease rises to the level of 

 an epizootic until an equilibrium has been established. 



The same is true of human diseases, among which smallpox is a 

 conspicuous example. The great pandemics of influenza, which seem 

 to travel from east to west every one or two decades, soon give way 

 to sporadic cases, and the careful work of many bacteriologists would 

 indicate that the influenza bacilli found at present have fallen to the 

 level of secondary invaders, and are parasites of the respiratory tract 

 in many affections. 



As pathogenic micro-organisms differ not only in the degree of par- 

 asitism attained, but also in their essential nature, a great variety of 

 diseases is the result. In a crude way they may be arranged into three 

 classes: 



(1) Micro-organisms which live upon the skin and the mucous 

 membranes, and invade the body only when lesions exist in these 

 structures, or where the general resistance is impaired. 



(2) Micro-organisms which appear only occasionally from some 

 unknown but permanent focus. They produce epidemics often highly 

 fatal, but they are successfully pushed back, because the strain cannot 

 readily adapt itself to the new conditions. 



(3) Micro-organisms which are most highly adapted for a parasitic 

 existence, and which produce diseases of a relatively fixed type. 



As regards the first class, the conditions under which they produce 

 disease rise more and more into prominence. The factor microbe 

 becomes almost secondary to other factors. Many of our most com- 

 mon diseases obey certain meteorologic laws. Thus diphtheria and 

 pneumonia are chiefly winter diseases, because the conditions of 

 throat and lungs which favor them are largely due to cold weather, 

 or, we might say, the cold weather acting upon an indoor sedentary 

 population, or one subjected to untoward influences, injures the re- 

 spiratory tract. Some microbes of this class depend upon the prepara- 

 tion made for them by others. Thus the exanthematous diseases, 

 such as scarlatina and smallpox, are frequently associated with or 

 followed by the invasion of streptococci, and the majority of deaths 

 are due to such secondary invasion. The streptococci live upon the 

 mucous membranes, and whenever the proper opportunity comes 

 they invade more vital territory. This group of bacteria is the fre- 

 quent cause of death in chronic diseases. Some years ago Professor 



