Complex sentence: Currently, Willett is the Fredrick John Stare Professor of Epidemiology and Nutrition, and Chair of the Department of Nutrition at Harvard School of Public Health, and is a Professor of Medicine at Harvard Medical School.
Split into simpler sentences: Currently, Willett is the Fredrick John Stare Professor of Epidemiology and Nutrition, and Chair of the Department of Nutrition at Harvard School of Public Health. He is also a Professor of Medicine at Harvard Medical School.