86 ILLINOIS ACADEMY OF SCIENCE 



three months of the year when a great epidemic of in- 

 fluenza was raging. The excess in number of deaths due 

 to influenza and pneumonia for the year was almost 

 exactly equal to the excess in general mortality. 



In speaking of the mortality rate for individual dis- 

 eases it is customary to speak of the rate per 100,000 of 

 population. During the year 1918 the rate in the United 

 States was 298.9 for influenza, and 284.3 for pneumonia, 

 all forms. This rate is not as large as the rates which 

 will be given for the State of Illinois, but it must be re- 

 membered that the year 1918 closed while the epidemic 

 was in full sway. The statistics which are studied for 

 Illinois are for the fiscal year of the State beginning 

 July 1, 1918 and closing June 30, 1919. These data for 

 the State of Illinois, therefore, cover the entire period of 

 the epidemic. 



For the annual report of the Department an estimate 

 was made, county by county, of the cost of certain com- 

 municable diseases. For influenza we estimated the cost 

 of treatment for each case at $10 and the loss of time at 

 $15. Each funeral was estimated at $100 and each life 

 lost was valued at $3,000. This very moderate estimate 

 showed an economic loss in the State of Illinois of $73,- 

 710,000, or an equivalent to a per capita loss of $11.59 to 

 each resident of the state during the single year, 1918-19. 

 The fact that a disease is communicable indicates that 

 theoretically it can be controlled; but to control it we 

 must know the conditions favorable. Accordingly a spe- 

 cial study was undertaken relative to this disease in the 

 State. 



In making a statistical study of influenza in Illinois we 

 must constantly bear in mind the sources of possible 

 error. 



1. Estimated population. The Census in Illinois is 

 taken on decennial years, and the population for inter- 

 mediate years is estimated according to previous growth. 

 Unusual development or decline in population is not ex- 

 pressed in these estimates. In other words the estimated 

 population for any area may be falsely large or small. 



