Pathology, puh THOL uh jee, is the study of disease, or any condition that limits the power, length, or enjoyment of life. Comparative pathology compares human diseases with those of animals. Human pathology is a branch of medicine. Pathologists use modern instruments and methods, such as electron microscopy, to recognize the changes caused by disease in the tissues and organs of the body. They try to explain why a diseased body acts differently from a normal body.