There are a number of symptoms associated with Crohn's disease that can vary from mild to severe. The most significant is inflammation of tissue within the gastrointestinal tract, most commonly in the lower part of the small intestine (called the ileum) or parts of the large intestine (known as the colon).

Inflammation is an immune response caused when the body determines it has been infected with an illness. It’s a way to destroy a potentially dangerous virus, bacteria, or other foreign object perceived as threat.

Though the cause of Crohn's is not known for certain, experts believe the immune system may be over-reacting either to a real threat from an unidentified virus or bacteria, or to the benign bacteria that exists in everyone's gastrointestinal system. It’s also possible that the body is perceiving the food it’s digesting as a threat, and is attempting to protect itself. 

This inflammation can lead to a host of other symptoms, including:

  • bleeding
  • diarrhea
  • ulcers
  • vomiting
  • cramps
  • unwanted weight loss
  • trouble absorbing the nutrients from digested food

In some cases, symptoms can be severe. They can include holes in the intestines and infection.  People who suffer from Crohn's disease also have a heightened risk of colon cancer.

When inflamed, the tissue affected by Crohn's disease is prone to bleeding. This can happen when the food being digested comes into contact with the inflamed areas, or the tissue can just bleed on its own. This can cause anemia and fatigue if it’s significant enough.

Bleeding can be confirmed through observing blood mixed with stool, or in the toilet bowl. Sometimes, the bleeding remains internal and requires specific tests to confirm the problem’s existence.

The inflamed tissue also flushes an excess of water and salt into the digestive tract. This mixes with stool to cause diarrhea. For some people with Crohn's, this can be a significant problem, as it creates a very frequent need to use the bathroom. 

Frequent diarrhea also creates a risk of dehydration because of loss of fluids.

Swelling and scarring can occur due to the inflammation, and this can lead to blockages in the intestines. This, in turn, can lead to cramping and vomiting.

Depending on what areas Crohn's affects, it can prevent the body from absorbing nutrients as it digests food. This can stem directly from the inflammation or it can be caused by a hole in the intestines called a fistula. 

Crohn's disease can cause ulcers (open sores) in the digestive system. If they are severe enough, these ulcers can eat completely through tissue, creating the fistula. “Fistulas are commonly associated with diseases such as inflammatory bowel disease (IBD) and Crohn's disease, and are communications between one part of the body to another,” said Charles T. Richardson, M.D., a gastroenterologist with Baylor University Medical Center in Dallas, Texas. “In Crohn’s disease, these communications can occur from the bowel to the skin, from one loop of bowel to another, and from the rectum to the skin, to name a few,” “Fistulas can occur from the bowel to the vagina in ladies and for men, from the bowel to the urinary bladder.”

Fistulas can cause malnutrition by allowing foodstuffs to take shortcuts through the digestive tract, thus preventing proper absorption of nutrients. They can also create pockets of infection called abscesses. Depending on their severity, fistulas and the infection they can cause can be life-threatening.

Researchers have found that having Crohn's disease can substantially increase a person's risk for colon cancer, so people with Crohn's must take extra care to be monitored for it.

Other symptoms of Crohn's disease can include:

“One serious event is small bowel or small intestine obstruction,” said Dr. Richardson. “The obstruction of the small bowel can occur because of strictures that occur in the small intestine as result of Crohn’s disease. Sometimes that needs to be treated surgically. In some cases, it can be treated medically. A frequent cause of surgery in patients with Crohn’s disease is small intestinal obstruction.”