Just something to think about.

one is a chronic conditions (really a seceptability), the other is an acute attack.

attentions to the chronic condition, (avoiding seeds, etc) tend to reduce the likelyhood of the acute version.

One small additional note, of troy and WW. The -osis refers to the condition itself, in this case diverticula. Whilst the -itis refers to inflammation *in or of the diverticula. Inflammation could be both acute and chronic.

The wall of the intestine has three layers. The middle layer is the only one with much strength. Where there is a defect in the middle layer, the small amount of pressure of the intestinal contents can cause an outpouching of the wall, a diverticulum. There may be a number of them, with no symptoms. That's diverticulosis. But indigestible fiber, for instance, may lodge in a diverticulum, and cause inflammation, sometimes very troublesome. That's diverticulitis.

Diverticulosis, having a bunch of them, is generally bland. Annoying sometimes, but not dangerous. Occasionally they cause a problem by bleeding.

Diverticulitis (as has been stated) is an inflammation, with fever, pain, occasionally even perforation and peritonitis, and can be quite serious indeed.