German bacteriologist who, with Edwin Klebs (Klebs, Edwin), in 1884 discovered the organism that causes diphtheria, Corynebacterium diphtheriae, commonly known as the Klebs–Löffler bacillus. Simultaneously with Émile Roux and Alexandre Yersin, he indicated the existence of a diphtheria toxin. His demonstration that some animals are immune to diphtheria was a basic feature in Emil von Behring's work in antitoxin development.

The son of an army surgeon, Löffler studied medicine at Würzburg University and at the Friedrich Wilhelm University in Berlin before serving in the army during the Franco-German War (1870–71). He obtained his medical degree at Berlin in 1874 and, after a period of service as an army doctor, became an assistant in the Imperial Health Office (1879–84), Berlin, where he was an associate of Robert Koch. He was professor of hygiene from 1888 at the University of Greifswald, where he served as rector from 1903 to 1907, and in 1913 he became director of the Robert Koch Institute for Infectious Diseases in Berlin.

Löffler also discovered the cause of swine erysipelas and swine plague (1885) and, with Wilhelm Schütz, identified the causative organism of glanders, Pfeifferella (Malleomyces) mallei (1882). With Paul Frosch he found that foot-and-mouth disease is caused by a virus—the first time the cause of an animal disease was attributed to a virus—and developed a serum against it.