Text Appearing Before Image: 394 ANIMAL BIOLOGY Most inspiring is the long story of heroism and hard work which has made it possible to cope with yellow fever; made possible the building of the Panama Canal, since the earlier attempt by France was unsuccessful largely on account of its ravages. We even have to be reminded that half a million cases occurred in the United States during the past century: the epidemic of 1793 took a total of one-tenth of the population of Philadelphia, and that of 1878 killed more than thirteen thousand in the Mississippi Valley alone. Text Appearing After Image: Fig. 248. — Results of a quarter century of yellow fever control. (From the Rockefeller Foundation.) However, while this and more is true, the recent discovery of new sources of infection in the interior of South America and Africa has made the problem much more complex and the complete control of the fever less certain than it appeared to be a decade ago. (Fig. 248.) Syphilis. The brilliant investigations, chiefly of the proto- zoologist Schaudinn in 1905, revealed the unicellular parasite, Treponema pallidum, that is the cause of syphilis — one of the greatest scourges of mankind since it became widespread during the sixteenth century. The ravages of the parasite produce many symptoms — frequently a type of paralysis, or paresis, with a
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