People who are infected with the E. coli strain O157:H7 and have bloody diarrhea sometimes are suspected of having other problems, such as:

• Infectious diarrhea (gastroenteritis) caused by other bacteria, viruses, or one-celled microorganisms called protozoa.
• Inflammatory bowel disease (Crohn's disease and ulcerative colitis). These long-lasting (chronic) diseases cause bloody diarrhea.
• Inflammation of the colon caused by sudden blockage of blood vessels (ischemic colitis), which is more common in people age 65 and older.
• A bleeding stomach ulcer.
• An inflamed appendix (appendicitis).
• Intussusception, a disorder in children in which one part of the intestine folds into itself. The main early symptom is severe abdominal pain.

Diagnosis can be further complicated because diarrhea caused by E. coli O157:H7 is not always bloody. In addition, a doctor may not consider E. coli O157:H7 as the cause of symptoms because bacterial infections are usually accompanied by a fever; a person infected with E. coli O157:H7 usually does not have a fever or has only a mild fever.