Pfeiffer is remembered for his many fundamental discoveries in immunology and bacteriology, particularly for the phenomenon of bacteriolysis. In 1894 he found that live cholerabacteria could be injected without ill effects into guinea pigs previously immunised against cholera, and that blood plasma from these animals added to live cholera bacteria caused them to become motionless and to lyse. This could be inhibited by previously heating the blood plasma. He called this bacteriolysis and it became known as the Pfeiffer Phenomenon, or Isayev-Pfeiffer phenomenon. 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. Pfeiffer was a pioneer in typhoidvaccination. He discovered the specific bacteria-dissolving immune bodies in cholera and typhus. The British pathologist Almroth Wright is generally credited with the initiation of typhoid vaccination in 1896. His claims of priority were challenged as early as 1907 in favour of Richard Pfeiffer. A review of the original literature of the 1890s and the early 1900s revealed that several groups were working on typhoid vaccine at the same time and that the credit for the initiation of typhoid vaccine studies should be shared by these two great researchers. In 1892 he isolated what he thought was the causative agent of influenza. The culprit, according to Pfeiffer, was a small rod-shaped bacterium that he isolated from the noses of flu-infected patients. He dubbed it Bacillus influenzae, which was later called Haemophilus influenzae. 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. When history's deadliest influenza pandemic began in 1918, most scientists believed that Pfeiffer's bacillus caused influenza. With the lethality of this outbreak 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. In many patients, but not all, the bacteria were found. Failures to isolate B. influenzae were generally ascribed to inadequate technique, as the bacteria were notoriously difficult to culture. The first blow to Pfeiffer's theory came from Peter Olitsky and Frederick Gates at The Rockefeller Institute. Olitsky and Gates took nasal secretions from patients infected with the 1918 flu and passed them through Berkefeld filters, which exclude bacteria. The infectious agent — which caused lung disease in rabbits — passed through the filter, suggesting that it was not a bacterium. Although the duo had perhaps isolated the influenza virus, other researchers could not reproduce their results. In 1896 he isolated micrococcus catarrhalis that is the cause of laryngitis. M catarrhalis also causes bronchitis, and pneumonia in children and adults with underlying chronic lung disease. It is occasionally a cause of meningitis. Richard Pfeiffer also invented a universal staining for histological preparations. Pfeiffer studied at the Kaiser-Wilhelms-Akademie in Berlin from 1875 to 1879. After completing his studies he was conferred doctor of medicine in 1880 and subsequently served as an army physician and bacteriologist until 1889. He was a student of Robert Koch, and from 1887 to 1891 worked as Koch's assistant in the Institute of Hygiene in Berlin. In 1891 he was entrusted with the leadership of the scientific department of the Institute of Infectious Diseases in Berlin. In 1897 Pfeiffer joined the German expedition under Robert Koch to India to investigate the plague. The following year he went to Italy with Koch to do research on malaria. He moved to Königsberg to enter the chair of hygiene in 1899, succeeding Erwin von Esmarch. He remained in that city until 1909, when he moved on to the same chair in Breslau. Pfeiffer retired there as emeritus in 1925.
