Hi meg, and welcome. I don't really know the answer to your question, but in my experience I think it can spread and get worse. But, I was diagnosed with left sided colitis and about
45 cm of my colon affected. It is now 10 years later and it is holding steady. I think once you start treatment, the likelyhood of it spreading decreases tremendously. At least this has been my experience.....I am sure others will weigh in on your question and correct me if I am wrong. Best of luck.