Ulcerative colitis. Coloured scanning electron micrograph (SEM) of ulcerative colitis (UC) showing vascularity (blood flow). The cause of this disease is thought to be to do with the body's immune system's reaction to a virus or bacterium. It occurs in the colon and is characterised by inflammation in the intestinal wall leading to diarrhoea, abdominal pain, and rectal bleeding. UC is chronic (ongoing) and in extreme cases can require surgery to remove the colon. Magnification: x235 when printed 10 cm wide.