Sarah DeBord was 24 when she first noticed blood in her stool. She saw a gastroenterologist who did a procedure called a flexible sigmoidoscopy to examine her lower colon and rectum, then gave her a diagnosis of hemorrhoids.

“I don’t see any hemorrhoids, but that’s probably what it is,” Ms. DeBord recalled the doctor saying.

For the next 10 years, whenever Ms. DeBord saw blood in her stool, she told herself it was probably hemorrhoids. At 34, she began losing a lot of weight, which she at first chalked up to her training for a half marathon. But she was also so constipated she was making endless trips to the bathroom. She had a colonoscopy and learned she had advanced colorectal cancer that was inoperable and had spread to her lung.