DISTRIBUTION OF PATHOGENIC BACTERIA 111 
have a marked tendency to occur at definite seasons of the year. This 
tendency is rendered all the more plausible because the seasonal dis- 
tribution north and south of the equator coincides rather closely with 
the fact that the seasons are six months apart in the northern and south- 
ern hemispheres respectively. Thus, midwinter in the northern 
hemisphere is midsummer in the southern. 
There is some evidence, also, that this seasonal incidence of specific 
disease corresponds rather closely with periods of increase and decrease 
in the causative organisms themselves. Thus, Boycott' found in an 
examination of some 15,000 throat cultures in England, extending over 
a period of six years, that diphtheria bacilli were most abundant in 
the months of September, October and November, the period when the 
disease diphtheria was most prevalent. On the other hand, the rela- 
tively avirulent Hofmann bacillus, found under identical circumstances, 
was most commonly found in maximal numbers during the months 
from ]May to August inclusive.- The causes which lead to this irregu- 
larity of occurrence are unknoAVTi. 
Certain diseases which are, or may be transmitted by droplets, as 
plague and influenza, likewise occur in pandemic form, spreading with 
great rapiflity from place to place, apparently along the channels of 
communication. These world-wide waves of infection seem to rise 
suddenly, spread rapidly, and in the case of influenza at least, disappear 
quickly, although successive waves, frequently more severe, may follow 
the first. 
Another form of periodic or recurrent disease of microbic causation 
is in association with the intestinal group. Epidemics of bacillary 
dysentery haA'e been reported from time to time in Japan, Germany, 
and on the Atlantic seaboard of the United States. There are inter- 
vening years in which dysentery cases are few or almost absent. Such 
an occurrence has been found in the United States'' in which five years 
elapsed between the periods of occurrence of dysentery bacilli in 
significant numbers. 
One of the great problems of public health is the study of the biology, 
or natural history of epidemic disease, and the unfolding of the factors 
which are invohed in the occurrence of these devastating microbic 
visitations from time to time. 
Epidemiology.'*— The separation of those bacteria which are habitu- 
ally associated with man into two groups, the parasitic and the 
pathogenic types, emphasizes on the one hand difl'erences in their 
biology and focusses attention on the other hand upon the epidemio- 
logical and preventive aspects of their life cycles. 
The typical parasitic organisms, as staphylococci, are normal resi- 
' Jour. Hyg., 1905, 5, 22.3. 
- Literature reviewed by Graham-Smith: Ibid., 1903, 3, 21(); 1904, 4, 2.5s. 
•^ See Kendall: Seasonal Recurrence of Intestinal Infection, Southern Med. .Jour., 
1925, 18, 120. 
* See Kendall: Nation's Health, 1926, 8, No. 2. 
