The factors that trigger intestinal inflammation and onset of IBD remain unknown. Symptoms of IBD may develop suddenly or gradually and include constipation, diarrhea, fever, rectal bleeding, and abdominal discomfort. In both Crohn disease and ulcerative colitis, patients may experience periods of symptom remission and relapse. IBD is particularly difficult to diagnose in children, and affected children may fail to grow properly. In addition to physical examination, blood tests, and stool analysis, IBD may be diagnosed by colonoscopy, in which the entire colon is investigated, or sigmoidoscopy, in which only the rectum and sigmoid colon are investigated. Individuals with a family history of IBD may undergo genetic testing for specific gene mutations (mutation) to determine their susceptibility to the disease.

Research has indicated that IBD is polygenic, meaning that variations in multiple genes (gene) combine to give rise to the disease. While the exact combinations of genetic variants that cause IBD have not been identified, individual genetic variations associated with the disease have been discovered. For example, mutation of a gene called TNFRSF6B (tumour necrosis factor receptor superfamily, member 6b, decoy), which is involved in suppressing inflammation in the gastrointestinal tract, has been linked with the onset of IBD in childhood. In addition, variation of a gene called GLI1 (glioma-associated oncogene homolog 1), identified in patients in northern Europe, results in reduced activity of the anti-inflammatory GLI1 protein, and dampened activity of this protein has been associated with increased inflammation in the intestines. Furthermore, some genetic variations are associated with increased risk for both ulcerative colitis and Crohn disease, whereas other variations are disorder-specific. For example, variation of a gene called ECM1 (extracellular matrix protein 1) has been linked to ulcerative colitis, whereas variation of a gene called NOD2 (nucleotide-binding oligomerization domain containing 2) has been linked to Crohn disease.

The discovery of genetic mutations that lead to specific abnormalities in immune function in IBD has facilitated research into the development of unique treatment strategies. For example, there is potential for the development of an agent that targets the GLI1 protein to restore the protein's activity to normal levels, thereby reducing intestinal inflammation and relieving symptoms.

Treatment generally includes a diet low in fat, high in protein and easily digestible carbohydrates (carbohydrate), and free of lactose (milk sugar). Increased intakes of certain nutrients, such as iron, calcium, and magnesium, and supplementation with fat-soluble vitamins (vitamin) may also be recommended, along with additional fluid and electrolytes (electrolyte) to replace losses due to diarrhea. Anti-inflammatory agents, such as corticosteroids (e.g., prednisone) and mesalamine, and immunosuppressive agents, such as cyclosporine and methotrexate, may be prescribed for patients with moderate to severe IBD.