Ulcerative colitis (UC) is a chronic disease characterized by acute episodes of bloody diarrhea with varying degrees of severity. The most feared event, acute severe UC, can lead to life-threatening and systemic complications. Little is known about the determinants of severity in UC. The investigators hypothesize that distinct severity phenotypes of UC arise from intricated host-microbiota mechanisms influencing repair mechanisms. Our first and main objective is to study the natural history of UC during a one-year time period in a cohort of patients.