Guillaume de Baillou was born in 1538, the son of a famous mathematician, architect, and engineer. His affluent family owned an estate at Nogent-le-rotrou. He studied at the University of Paris, concentrating in Latin, Greek, and Philosophy, and later earned a Bachelor of Arts degree in 1568. He continued his studies there, earning a Doctorate in Medicine in 1570.

The first epidemiologist since Hippocrates, Baillou studied the epidemics of Paris between 1570 and 1579. He is known for his descriptions of the plague, measles, and diphtheria. He also taught the humanities, and later became professor of medicine. Baillou served as a teacher for 46 years, eventually becoming Dean of the Faculty of Medicine. Refusing to leave his medical practice, he declined King Henry IV's invitation to be a physician to the Dauphin, the eldest son of the French king, as Baillou was esteemed for his treatment of children. Baillou, however, did later became a physician to Henry IV. He died at the age of 78 in Paris in 1616.