Guillaume De Baillou was a French physician and epidemiologist and studied under Fernel.  He is remembered as the first epidemiologist since Hippocrates and the founder of modern epidemiology. He was a prolific writer, but his work was published posthumously. His most important publications are :

* Epidemiorum et ephemeridum libri duo     
* Consiliorum medicinalium  libri II             
* Liber de rheumatismo et pleuritide dorsale  
* De virginum et mulierum morbis liber         
* Definitionum medicinarum liber                   

In the 16th century, Baillou studied epidemics that were devastating Paris. He provided an excellent description of plague, measles, and diphteria which was published in "Epidemiorum et ephemeridum libri duo."  He also provided the first description of whooping cough (Tussis quintana) in 1578.

He also wrote one of the first accounts on pericardial effusion and pericarditis with congestive heart failure (or so-called dropsy). These reports were published in his treatise "Consiliorum medicinalium  libri II."  Morgagni gave credit to Baillou in his famous "De sedibus"  for describing heart failure as one of the complications of pericarditis.

Finally, Baillou introduced  the term  rheumatism in its modern sense. In his description of the disease, he distinguished between acute and chronic forms of polyarthritis.