Paul-Louis Simond was born in Beaufort-sur-Gervanne, France, on July 30, 1858. He was an assistant in natural history at the School of Medicine and Pharmacy in Bordeaux (1878-1882) and director of the Acarouani leprosarium in French Guiana (1882-1886), the Pasteur Institute in Saigon (1898-1901), and the Imperial Institute for Bacteriological Research in Constantinople (1911-1913). In 1887, he received his medical degree in Bordeaux with a thesis on Hansen’s disease. After working as a doctor for the French Navy, he joined the Pasteur Institute in Paris in 1895. He fought smallpox and the bubonic plague in Asia and made seminal contributions to the biology of Coccidia. In 1898, he demonstrated the role of the rat flea in transmitting the plague, a discovery that earned him major scientific acclaim. Along with Marchoux and Salimbeni, he was a member of the Pasteur Institute mission that traveled to Brazil to investigate yellow fever from 1901 to 1905. He also studied the illness in Martinique (1908-1909). He passed away in Valence, France, on March 18, 1947.