A 37-year-old farmer complains about general weakness, spastic pain in the lower parts of his abdomen, mainly in the left iliac area, frequent defecations up to 18 times a day, feces contain admixtures of mucus and blood. The illness began abruptly 3 days ago with chill, fever, headache. General condition is moderately severe, body temperature is 37,8°C. Sigmoid colon is spasmed and painful. What is the most probable diagnosis?