Flare-up of ulcerative colitis (UC) is characterized by the inflammation of colon mucosa that requires the use of immunosuppressive therapies. In previous studies, the active role of cytomegalovirus (CMV) has been demonstrated, with a correlation between the cytomegalovirus DNA ((deoxyribonucleic acid) load in the inflamed tissue and the resistance to successive lines of immunosuppressive therapy (Roblin et al., Am J Gastroenterol 2011). The main aim of this monocentric prospective study is to evaluate the DNA viral load by qPCR (Polymerase Chain Reaction) of 3 herpesviruses (Epstein-Barr virus, herpes virus 6 and herpes simplex) together with that of CMV in colonic mucosa depending of the local inflammation (endoscopically normal region, inflamed mucosa or ulcer) in patients suffering of moderate to severe UC flare-up (Mayo score >6 with endoscopic score higher or equal than 2). The viral load will also be correlated to the Mayo endoscopic score and the response to immunosuppressive drugs (steroid and anti-TNF (Tumor Necrosis Factor) monoclonal antibodies).