Yea, our bodies regularly develop cancers, but they tend to be benign at worst. A healthy body can normally identify and destroy them. It's when the immune system is exhausted due to stress, malnourished or severe injury, that a normally easy to spot cancer develops into one the body cannot identify as a threat where someone develops malignant cancerous growths. I remember reading some medical study being done that attacks tumors in such a way as to enable the immune system to find and destroy them without things like chemo "therapy".