There are many causes of primary<unk> , or tumorigenesis. Most human cancers in the United States are caused by external factors, and these factors are largely avoidable. These factors were summarized by Doll and Peto in 1981, and were still considered to be valid in 2015. These factors are listed in the table. + External factors in cancer Factor Estimated percent of cancer deaths Diet 35 Tobacco 30 Infection 10 Reproductive and sexual behaviora 7 Occupation 4 Alcohol 3 Sunlight (UV) 3 Pollution 2 Medicines and medical procedures 1 Food additives a Reproductive and sexual behaviors include: number of partners; age at first menstruation; zero versus one or more live births
Malignant transformation
Tumor progression
Degenerative disease
Pre-existing condition
Psychogenic disease
Malignancy
Benign tumor
Viral disease