IBD affects an estimated 3 million Americans. Unfortunately the causes of IBD are not yet well understood. What we do know is that women and men are equally likely to be affected by IBD and that although IBD can occur at any age, it is most prevalent in people between the ages of 15 and 35. 

IBD occurs when the gut immune system is activated to attack intestinal bacteria with inflammation, however the immune response and inflammation persists. This chronic inflammation causes IBD symptoms.

Three factors have been identified to be be correlated with increased risk for developing IBD: genetics, gut barrier influences (damage from NSAIDs or viral or bacterial infections), environment (frequent use of antibiotics, smoking cigarettes). 

References:

1. https://www.crohnscolitisfoundation.org/what-is-crohns-disease/causes

2. https://www.crohnscolitisfoundation.org/what-is-ulcerative-colitis/overview