To explain why some people get a particular cancer and others do not, or why cancer is more prevalent in one society than in another, medical investigators known as epidemiologists painstakingly compare the dissimilar groups. The techniques of epidemiology, based on comparative medical examinations, health records and interviews, have dominated much of 20th century cancer research - but the seeds of these techniques were planted nearly three centuries ago. In 1700, the Italian doctor Bernardino Ramazzini noted that nuns - who lead lives very different from those of the general population - have a high susceptibility to breast cancer. He concluded - correctly - that it was in some way related to their celibacy. Among the first to use the techniques was an 18th century english surgeon, Percival Pott. A prominent figure in the london of his day, Pott treated such patients as the actor David Garrick and the man of letters Samuel Johnson. He was also a medical scholar, with interest ranging from hernias to broken legs. In 1775 he turned his attention to a curious phenomenon among london chimney sweeps. At an early age and to a degree far beyond that of their contemporaries, they were susceptible to cancer of the scrotum.

In "A Short Treatise of the Chimney Sweeper's Cancer", Pott described the work of these boys and young men. "Thrust up narrow and sometimes hot chimnies, where they are bruished, burned and almost suffocated; and when they get to puberty, become peculiarly liable to a noisome, painful and fatal disease. To get through those narrow chimneys, they often worked naked. They rarely bathed - the practice was considered unhealthful - and soot lay thick upon their skins, especially in the groin area". That soot - Pott pointed out - represented a substance peculiar to the chimney sweep's trade. In a bold leap of speculation, he declared that the soot was the cause of their cancer.

Pott could never prove his case, but his primitive methods became the foundation of modern cancer epidemiology. This medical specialty now offers the best opportunities for learning what causes cancer in human beings. Like Pott,epidemiologist have linked specific occupations and chemicals to particular cancers; similar studies have implicated diet, sexual habits and heredity as well.

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