Paul Ehrlich (1854–1915) was born near Breslau—then in Germany, but now known as Wroclaw, Poland—and studied to become a medical doctor at the university there and in Strasbourg, Freiburg im Breisgau, and Leipzig. In Breslau, he worked in the laboratory of his cousin, Carl Weigert, a pathologist who pioneered the use of aniline dyes as biological stains. Ehrlich became interested in the selectivity of dyes for specific organs, tissues, and cells, and he continued his investigations at the Charité Hospital in Berlin. After he showed that dyes react specifically with various components of blood cells and the cells of other tissues, he began to test the dyes for therapeutic properties to determine whether they could kill off pathogenic microbes.
After his own bout with tuberculosis—probably contracted in the laboratory—and his subsequent cure with Robert Koch's tuberculin therapy, Ehrlich focused his attention on bacterial toxins and antitoxins. At first, he worked in a small private laboratory, but as the quality of his work became recognized by Koch and others, he was able to command more and better resources—eventually at the State Serum Institute in Frankfurt. In 1908, he received the Nobel Prize in medicine for his work on immunization.
In Frankfurt, he continued to look for chemical agents to use against disease. He obtained the cooperation of the nearby Cassella chemical works, which donated samples of new compounds produced in their laboratories for him to test for biological activity. In 1906, Georg-Speyer-Haus, a research institute for chemotherapy, was established with its own staff under Ehrlich's direction. The research programs were guided in part by Ehrlich's theory that the germicidal capability of a molecule depended on its structure, particularly its side-chains, which could bind to the disease-causing organism. The most successful products of this quest were Salvarsan (1909–1910)—dihydroxydiaminoarsenobenzenedihydrochloride—and Neosalvarsan (1912), the most effective drugs for treating syphilis until the advent of antibiotics in the 1940s.