492 PATHOGENIC MICROORGANISMS 



secondary waves that lasted as long as 1800, and similar observations 

 were made during the epidemics of 1836 and 1837. The most careful 

 studies of such waves were made by Parsons during the 1889 

 epidemic. He divided this epidemic into the first wave which began 

 in England in the winter of 1889 to 1890, a second wave in the 

 spring of 1891, and a third in the winter of 1891 to 1892. Frost 

 has similarly studied the American epidemics, and has come to 

 analogous conclusions. Brownlee 19 has attempted to establish a law 

 of periodicity for the intervals between pandemics, and for the 

 intervals between several waves of each outbreak. In general, his 

 studies seem to show that there is an approximate period of ten 

 years between large epidemics, and that a period of about thirty- 

 three weeks intervenes between individual waves. We cannot go 

 into these purely statistical facts in the present work, but refer 

 the reader to papers by Brownlee, and the more recent paper by 

 Raymond Pearl. 20 



Secondary and tertiary waves have certain characteristics which 

 it is important to note. In contrast to the relatively mild onset 

 of the primary waves, these later waves are marked by greater 

 severity of the cases, and almost universal secondary infection. The 

 disease takes on a much more dangerous respiratory form. The 

 mortality becomes progressively higher during these waves than 

 during the original outbreak. Leichtenstern, also agrees that, in 

 the secondary and tertiary waves, that the morbidity is lower and 

 the mortality much higher. Similar observations have been made 

 by Wutzdorff 21 for the 1889 epidemic. 



The secondary epidemic waves do not travel with the same speed 

 and to the same extent as do the first waves. Cases are more scat- 

 tered and the period of prevalence is more prolonged. These waves 

 never stop abruptly, but play out, in gradually diminishing ripples, 

 into subsequent years. Also, according to Leichtenstern, these 

 secondary and tertiary waves do not seem to take their origins 

 from a single place, but crop up here and there from many scattered 

 foci. As Netter says, "they have appeared in separate, synchronous 

 or successive explosions, without connection between various reap- 

 pearances in different places, as this was possible during the first 

 appearances in 1889." 



19 Krownlce, Lancet, 2, 1919, 856. 



= Pearl, E., U. S. Pub. Health Serv. Rep., No. 548, August 8th, 1919, 



21 Wutzdorff, Arb. a. d. k. Gesundh., 9, 1894, 414, 



