of the places that he had methodically can- 

 vassed to estabhsh the field of epidemiol- 

 ogy. I later reahzed that I had begun this 

 tour by walking out of the library. 



Snow moved to London in 1836; Dar- 

 win in 1837. hi the early 1840s Darwin 

 was hving on Gower Street, a block north 

 of where the London School of Hygiene 

 would be built. He was, in his words, "col- 

 lecting facts bearing on the origin of spe- 

 cies." At that time John Snow was work- 

 ing on his degree at the University of 

 London, which was across the street from 

 Darwin's apartment. But Darwin and 

 Snow apparently never met and may not 

 have even been aware of each other's 

 earth-shaking contributions. Although 

 Snow was four years younger than Dar- 

 win, he died of a stroke in 1858, at the age 

 of forty-five, one year before the publica- 

 tion of Origin of Species. Each time I went 

 to the hbrary that summer, I walked down 

 Gower Street, where Snow and Darwin 

 must have walked separately many times 

 during the early 1 840s. Chance had put me 

 in the same place, and the printed words in 

 the library had removed the barrier of 

 time, allowing a linkage between Snow's 

 epidemiology and Darwin's evolution. 



Isaac Newton paid homage to scientists 

 such as Gahleo and Copernicus by writ- 

 ing, "If I see farther than other people, it is 

 because I stand on the shoulders of gi- 

 ants." The rest of us also have the chance 

 to see farther if we do a httle giant climb- 

 ing. As for me, I was teetering with one 

 foot on Charles Darwin's shoulder and the 

 other on John Snow's. We cannot predict 

 precisely what new views will come from 

 the merging of epidemiology and evolu- 

 tion, but we can see many possibihties. A 

 better understanding of the evolution of 

 virulence should allow us to identify inter- 

 ventions that will not only reduce the 

 spread of infections but also force 

 pathogens to evolve to milder states by 

 making harmfulness too costiy for them. 

 Diarrheal pathogens may be forced into a 

 benign state by water purification. Vector- 

 borne pathogens may be similarly trans- 

 formed by the installation of mosquito- 

 proof housing that prohibits transmission 

 from severely ill people. I expect that sci- 

 entists at the end of the twenty-first cen- 

 tury will find it curious that today's health 

 scientists were so skilled at recognizing 

 the importance of molecular biology and 

 biochemistry, but that it took more than a 

 century after the birth of evolutionary biol- 

 ogy, epidemiology, and microbiology for 

 us to realize the importance of using evo- 

 lution as a tool for controlhng disease. 



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