Richard Friedrich Johannes Pfeiffer FRS[1] (27 March 1858 – 15 September 1945) was a German physician and bacteriologist.

Working with Robert Koch in Berlin he intellectually and experimentally conceived the concept of endotoxin as a heat-stable bacterial poison responsible for the pathophysiological consequences of certain infectious diseases.

Endotoxin and anti-endotoxin antibodies have since then fascinated researchers of many disciplines, particularly in the fields of diagnosis, prevention, and therapy of severe Gram-negative infections.

Few doubted the validity of this discovery, in large part because bacteria had been shown to cause other human diseases, including anthrax, cholera, and plague.

With the lethality of this outbreak (which killed an estimated 20 to 100 million worldwide) came urgency—researchers around the world began to search for Pfeiffer's bacillus in patients, hoping to develop antisera and vaccines that would protect against infection.

Olitsky and Gates took nasal secretions from patients infected with the 1918 flu and passed them through Berkefeld filters, which exclude bacteria.

After completing his education at the gymnasium in Świdnica, Pfeiffer studied medicine at the Kaiser Wilhelms Akademie in Berlin from 1875 to 1879.