EPIDEMIOLOGY OF PNEUMONIA 473 



10 per cent of the death rate, but under conditions like those occurring 

 during the war, a much larger percentage of all deaths are due to 

 pneumonias. The Surgeon General estimates that in the year 1918 the 

 total number of deaths chargeable to respiratory disease (and this 

 means with very few exceptions, death by pneumonia of one kind or 

 another) was 39,701, out of a strength of 2,518,499 men, which 

 amounts to a death rate of 15.75 per thousand, and 82 per cent of 

 all deaths occurring in the Army during this year. 



In order to discuss the epidemiological and preventive problem 

 concerned with pneumonia with intelligence, it will be necessary to 

 discriminate between the so-called "primary" pneumonias and "sec- 

 ondary" pneumonias. 



Inflammations of the lung may be caused by a variety of bacteria. 

 However, for the purposes of considering the epidemiology of these 

 diseases we need take into account only those caused by various pneu- 

 mococci, the hemolytic streptococci, and influenza bacilli. The charac- 

 teristics of an epidemic will vary considerably according to whether 

 the majority of the cases are typical lobar pneumonias, coming on with- 

 out previous illness, or whether most of the cases represent pulmonary 

 infection, secondary to a preceding attack of influenza or to measles. 

 Typical lobar pneumonia is almost regularly a pneumococcus infection 

 and this type of the disease is by far less fatal than the other. The 

 secondary pneumonias may be caused by many different organisms. 

 Even in the same community, cases occurring at about one and the 

 same time, may be caused by various pneumococci, or streptococci, the 

 majority of the cases being due to organisms most prevalent in that 

 particular place. Such epidemics of secondary pneumonia are the 

 types which are most apt to develop in times of war or under other 

 abnormal community conditions, and this type is far more fatal than 

 is the typical lobar pneumonia. 



Primary Pneumonias. That pneumonia was a communicable dis- 

 ease was recognized by Johannesen and other clinicians as early as the 

 middle of the last century. This point of view, however, was not 

 generally accepted until quite recently. One of the difficulties that has 

 stood in the way of a more general belief in the communicability of the 

 disease lias been the fact that many normal individuals harbor in 

 mouth and throat pneumococci which, until recent years, were indis- 

 tinguishable from the organisms found in the lungs in pneumonia. It 

 was taken for granted, therefore, that the entrance of pneumococci into 



