I is the key factor in cancer's growth in the human body. IGF-I is identical between humans and cows. Milk is a hormonal delivery system and IGF-I is orally active, surviving digestive processes. If you believe that breastfeeding mothers deliver substances to their infants, you then should understand that milk is a hormonal delivery system. Lactoferins, immunoglobulins and protein hormones survive digestion and exert powerful growth effects.
Cancer is very common, but usually controlled by the human body.
Cancerous tumors are the ones that have somehow thrown off the usually tight genetic controls on unwanted growth.