Born the son of a mining official in Klausthal, Germany, Koch studied medicine at the University of Göttingen where he was a pupil of Jacob Henle. After graduating in 1866 and serving in the Franco–Prussian War, Koch was appointed district medical officer in Wollstein. Here, working alone with only the most modest of resources, Koch began the research that was to make him, with Pasteur, one of the two founders of the new science of bacteriology.

Koch saw more clearly than anyone before what was involved in bacteriological research and achieved his first success with anthrax. Whereas Casimir Davaine had succeeded in transmitting anthrax from one cow to another by the injection of blood, Koch saw that the emerging germ theory required something more specific. It needed to be the germ that was injected rather than a fluid that could only be presumed to contain the organism. He thus spent three years devising techniques to isolate the anthrax bacillus from the blood of infected cattle and then to produce pure cultures of the germ.

By 1876 Koch was ready to publish the life history of the anthrax bacillus. He had found that while the bacillus in its normal state is somewhat sensitive and unable to survive long outside the body of its host, it forms resting spores, which are particularly hardy. Such spores, persisting in deserted ground, are responsible for the apparently spontaneous outbreaks of anthrax in healthy and isolated herds. Koch followed this with work on septicemia, during which he developed techniques for obtaining pure cultures.

With these triumphs behind him Koch at last achieved official recognition, being appointed to the Imperial Health Office in Berlin in 1880. In this office Koch, with the aid of his assistants Friedrich Löffler and Georg Gaffky, began one of the great periods of medical discovery. Much of this was based upon techniques of staining and culture growth developed by Koch in the obscurity of Wollstein. He developed culture media suitable for bacterial growth, proceeding from liquid media to boiled potato, to the still commonly used agar plates. Agar plates together with the new stains derived from aniline dyes constituted the heart of Koch's technique.

With them he made, in 1882, his most famous discovery – the bacillus responsible for tuberculosis TB. In the second half of the 19th century TB, responsible for one in seven of all European deaths, was the most feared of all diseases. The difficulties Koch faced were formidable. The bacillus was only about a third of the size of the anthrax bacillus, grew much more slowly, and in general was more difficult to detect. With great patience Koch managed to culture the thin rod-shaped bacilli, which he used to inoculate four guinea pigs. All four developed TB while two uninoculated controls remained uninfected.

The fame won by Koch for this work brought him into open competition with Pasteur, a competition fanned by Franco-German nationalism in the aftermath of the 1870 war. In 1883 both sides met in Egypt to study cholera. The French, under Emile Roux, seem to have mistakenly confused platelets in the blood with the vibrio responsible for cholera. Koch, noting microorganisms in the small intestines of the victims, took them to be the cause of the disease, an assumption confirmed when he observed the same comma-shaped rods in the intestines of Indian victims. At this point Koch was forced to violate his own rules because, although he failed to infect experimental animals with pure strains of the vibrio, he nonetheless declared the organism to be the cause.

In 1885 Koch was appointed professor of hygiene at the University of Berlin and in 1891 became head of the newly formed Institute of Infectious Diseases. The pressure on Koch to ‘earn’ this latter appointment led him to announce in 1890 the discovery of a substance he claimed could prevent the growth of tubercle bacilli in the body. The new drug, a sterile liquid containing dead tubercle bacilli, which he named tuberculin, was consequently in huge demand. However it had little effect in most cases and probably exacerbated some. It later proved useful however in testing whether patients have experienced tuberculosis infection, by noting their local reaction to an injection of tuberculin.

Koch resigned the directorship of the Institute in 1904 to become one of the first of the emerging breed of international experts. Indulging his passion for travel, Koch spent his last years advising South Africa on rinderpest, India on bubonic plague, Java on malaria, and East Africa on sleeping sickness.

In 1905 Koch was awarded the Nobel Prize for physiology or medicine for “his discoveries in regard to tuberculosis.” Perhaps as important as this, or any of his other specific discoveries, was his formulation of the so-called Koch's postulates. To establish that an organism is the cause of a disease we must, Koch declared, first find it in all cases of the disease examined; secondly it must be prepared and maintained in a pure culture; and finally it must, though several generations away from the original germ, still be capable of producing the original infection. Such postulates, first formulated fully in 1890, rigorously followed, established clinical bacteriology as a scientific practice.