The German professor Wilhelm Fabry believed that breast cancer was caused by a milk clot in a mammary duct. The first cause of cancer was identified by British surgeon Percivall Pott, who discovered in 1775 that cancer of the scrotum was a common disease among chimney sweeps.Photo Source: National Cancer Institute, Wikipedia Cancer cells in culture from human connective tissue, illuminated by darkfield amplified contrast, at a magnification of 500x.