First 20 days of calves in their lives and 20 days after weaning are very important periods regarding to calf pneumonia. Because generally calf diarrhea occurs in the first three weeks of calf's life and causes immune system malfunction and pneumonia agents can find the chance to infect calf. As we know they are in enviroment and always ready to cause disease. Pasteurella is the main cause of the pneumonia and generally works with other infective agents (virus, bacteria) and mix infections occurs. This can be prevented by using Pasteurella vaccines and hygienic precautions. After weaning calves are generally taken to group barns and different diets used in their feeding which causes stress and immune system supression and pneumonia cases generally increase in this period. Calves must be housed in small groups and feed changes should be done slowly and softly. If we manage these periods as best as we can, Pneumonia cases decrease at he same ratio.