My partner has diverticulitis, the first we knew about it was eight years ago. He had been feeling off colour for a few days, then one morning he rang me at work saying he was in terrible pain. I went straight home and after seeing the state he was in, I called 999.
The paramedics gave him gas and air for the pain and carted him off to the hospital, after a scan they discovered his bowel had actually perforated due to an abscess bursting in one of the diverticula pockets, he needed emergency surgery.
So it can be very serious. Thank goodness he has been OK since, he's had a couple of small flare ups requiring a course of antibiotics, but nothing really bad.