The good news is even if it was mimicking, he has no symptoms. Miracles happen? I'm glad his symptoms have improved.
What are the periods of flare-ups or remission for ulcerative colitis?...
"Ulcerative colitis has alternating periods of flare-ups and remission. During remission, the disease seems to have disappeared. The periods of remission can last from weeks to years."
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Springer - Mimics of Ulcerative Colitis...
"Infectious colitides such as Clostridia difficile, hemorrhagic E. coli, Campylobacter, Aeromonas, and Salmonellosis can often present as severe ulcerative colitis. Other infectious pathogens, which also need to be included in the differential diagnoses, include Amebiasis, Shigellosis, Cytomegalovirus, and Rotavirus. Inflammatory diseases of the colon which can present as UC include Crohn’s disease, ischemic colitis, radiation colitis, eosinophilic colitis, colorectal cancer, irritable bowel syndrome, and allergic reactions to medications such as mesalamine products, NSAIDs, and antibiotics."