Félix Étienne Pierre Mesnil (Omonville-la-Petite, La Manche department, 12 December 1868 – 15 February 1938, Paris) was a French zoologist, biologist, botanist, mycologist and algologist.He was a student of Alfred Giard at the École Normale Supérieure and a classmate of Maurice Caullery.He was an assistant of Louis Pasteur, and in the meantime, performed studies on cellular immunity, physiology and comparative pathology in the laboratory of Ilya Ilyich Metchnikov (1845–1916).He founded the Pasteur Institute Bulletin with Gabriel Bertrand (1867–1962), Alexandre Besredka (1870–1940), Amédée Borrel (1867–1936), Camille Delezenne (1868–1932) and Auguste-Charles Marie (1864–1935).In 1903, together with Alphonse Laveran (1845–1922), he showed that the parasite responsible for the visceral leishmaniasis (or "Kala-azar", a fever in India), first described by William Boog Leishman (1865–1926), is a new protozoa, different from Trypanosoma, the agent of the sleeping sickness, and from Plasmodium, the agent of paludism (malaria).