What are the odds of a 18 year old with all normal blood work and no family history having colon cancer? change in bm looser stool no other symptoms

Low: You are young, so risk is low, but young people could get it. With only symptoms of loose BM, I would first talks about irritable bowel, or change in diet. Depending on how long u had symptoms, and travel history- may be inflammatory disease or infections. But unlikely based on loose BM alone.

Colon cancer RARE: 90% of colon cancer is in people 50 and up. The average age of a patient with colon cancer is 72. The odds of an adolescent having colon cancer is 0.31 in 1 000 000. You should discuss your many concerns about colon cancer with your doctor. A simple stool test may help reassure you. Good luck.