The battle within

In general, IBD is thought to occur when the immune system overreacts to the bacterial community that normally dwells in the gut, setting in motion a chain of inflammation events that can damage and destroy the intestinal wall.

Similar, yet different

Credit: Illustration by Lucy Reading-Ikkanda

Most cases of IBD are classified as either ulcerative colitis (UC) or Crohn’s disease (CD). The relative prevalence of these two conditions varies widely across different geographical regions (see chart).

Between 5% and 15% of cases cannot immediately be put into either category and are instead referred to as ‘indeterminate colitis’. Despite sharing similar symptoms, there are important pathological distinctions between the two main forms of IBD.

Nature and nurture

A variety of hereditary, environmental and lifestyle factors are thought to influence IBD pathology. However, researchers are still grappling with exactly how these elements individually and collectively affect the natural history of the disease.

A growing global disease

IBD is most common in North America, Western and Northern Europe, Australia and New Zealand4. Cases are much less common in non-Western nations, but this is changing — incidence rates have climbed rapidly in recent years in parts of Asia and South America.

Regional, local, individual

Beneath the global data lie smaller-scale trends that reveal the complex mix of factors that determine a person’s risk of disease.

Credit: Illustration by Lucy Reading-Ikkanda
Credit: Illustration by Lucy Reading-Ikkanda


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