Diverticulitis is a condition where small pouches or pockets (diverticula) in the lining of the large intestine become inflamed and infected. The exact reason why diverticula develop is not known, but they are associated with not eating enough fiber. Fiber makes stools softer and larger, so less pressure is needed by the large intestine to push them out of the body. The pressure of moving hard, small pieces of stools through the large intestine creates weak spots in the outside layer of muscle, allowing the inner layer (mucosa) to squeeze through these weak spots, creating the diverticula.

When any of the diverticula become infected, it leads to symptoms of diverticulitis. Several factors may increase the risk of developing diverticulitis, including aging, obesity, smoking, lack of exercise, a diet high in animal fat and low in fiber, and certain medications such as steroids, opioids, and nonsteroidal anti-inflammatory drugs.

In summary, diverticulitis is caused by the inflammation and infection of diverticula in the large intestine, which develop due to weak spots in the outside layer of muscle caused by the pressure of moving hard, small pieces of stools through the large intestine. Not eating enough fiber is a key factor in the development of diverticula, and several other factors may increase the risk of developing diverticulitis.