148 THE SCIENTIFIC MOJIfTHLT
1465. These diseases Sudhoff regards as foci of an endemic spiro- chetosiS; which, in persons rendered weak and susceptible by wars, famine and debauchery, became a virulent infection. Sydenham saw European syphilis as a mode of West African yaws, and salvarsan is a true therapia sterUisans for the spirochete of yaws. Pasteur's law ex- plains the facts about the great plague of London (1666). When the disease began to abate, vast numbers of people who had fled the city returned, and Pepys, in his " Diary, made anxious predictions as to a possible recrudescence of the epidemic. But this was not the case. The plague had worn itself out, and it is said that some even occupied the beds of plague patients with immunity.^' Tet, while lack of sus- ceptible persons and attenuation of the specific virus are not identical causes, they may sometimes amount to the same thing. Since Parr's time, mathematical investigations of epidemics have followed two main lines. Brownlee, Greenwood and other English statisticians have ap- plied the skew curves, devised by Earl Pearson, to the analysis and gradation of the statistics of various epidemics, and Brownlee has found that most of the curves evolved are symmetrical bell-shaped curves of the Parr type, with the difference that the curves do not fall more rapidly than they rose, as in Parr's original hypothetical curve of 1866, but more slowly, as in the actual figures of his 1866 epidemic (Pear- son's type IV. curve).
Boss's investigations have followed the lines laid down by himself in 1904, and his ultimate aim is to account, not only for the epidemics which have a symmetrical or normal curve, but also for the asymmetry which characterizes many epidemics influenced by external forces. He divides infectious diseases into three classes: '^ (1) diseases like leprosy or tuberculosis, which vary little from month to month, but may slowly increase or decrease in the course of years; (2) diseases like measles, scarlatina, malaria and dysentery, which are constantly present in many countries and flare up as epidemics at frequent intervals; and (3) dis- eases such as plague or cholera, which disappear entirely after periods of acute epidemicity." Concerning the diseases of the second class, he inquires whether they may be due to " a sudden and simultaneous increase of infectivity in the causative agents living in infected persons, or to changes of environment which favor their dissemination from person to person, or merely to the increase of susceptible material in a locality due to the gradual loss of acquired immunity in the population there." It is known, for instance, that measles has occurred at Perth regularly every sixteen months during the last forty years, with but two varia- tions; in Glasgow, every fifteen and a half months up to 1800, and every twenty- four and a half months from 1855 on; while the London records of measles during 1840-1912, indicate a periodicity of about 1% years."
15 J. Brownlee, Proc. Soy. 8oc Edinh,, 1905-6, XXVI., 486. i«Bftt. M. J,, 1915, II., 652.
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