The small intestine is a winding, tightly folded tube about 20 ft (6 m)long in adults. It connects to the stomach on the top end and to the large intestine (colon) on the bottom end. Most of the food a person consumes is digested and absorbed in the small intestine. The lining of the small intestine has tiny, finger-shaped tissues (villi). These raised tissues greatly increase the surface area of the intestine, which absorbs calories and nutrients from food.
The small intestine consists of three parts. The first part, called the duodenum, connects to the stomach. The middle part is the jejunum. The third part, called the ileum, attaches to the colon.
- Stomach flu (enteritis): Inflammation of the small intestine. Infections (from viruses, bacteria, or parasites) are the common cause.
- Small intestine cancer: Rarely, cancer may affect the small intestine. There are multiple types of small intestine cancer, causing about 1,100 deaths each year.
- Celiac disease: An "allergy" to gluten (a protein in most breads) causes the small intestine not to absorb nutrients properly. Abdominal pain and weight loss are usual symptoms.
- Carcinoid tumor: A benign or malignant growth in the small intestine. Diarrhea and skin flushing are the most common symptoms.
- Intestinal obstruction: A section of either the small or large bowel can become blocked or twisted or just stop working. Belly distension, pain, constipation, and vomiting are symptoms.
- Salmonellosis: Salmonella bacteria can contaminate food and infect the intestine. Salmonella causes diarrhea and stomach cramps, which usually resolve without treatment.
- Shigellosis: Shigella bacteria can contaminate food and infect the intestine. Symptoms include fever, stomach cramps, and diarrhea, which may be bloody.
- Traveler's diarrhea: Many different bacteria commonly contaminate water or food in developing countries. Loose stools, sometimes with nausea and fever, are symptoms.
- Intussusception: Occurring mostly in children, the small intestine can collapse into itself like a telescope. It can become life-threatening if not treated.