His laboratory discovered arsphenamine (Salvarsan), the first antibiotic and first effective medicinal treatment for syphilis, thereby initiating and also naming the concept of chemotherapy.

He also made a decisive contribution to the development of an antiserum to combat diphtheria and conceived a method for standardising therapeutic serums.

As a schoolboy (inspired by his cousin Karl Weigert who owned one of the first microtomes), he became fascinated by the process of staining microscopic tissue substances.

He retained that interest during his subsequent medical studies at the universities of Breslau, Strasbourg, Freiburg im Breisgau and Leipzig.

After obtaining his doctorate in 1882, he worked at the Charité in Berlin as an assistant medical director under Theodor Frerichs, the founder of experimental clinical medicine, focusing on histology, hematology and colour chemistry (dyes).

[6] After completing his clinical education and habilitation at the prominent Charité medical school and teaching hospital in Berlin in 1886, Ehrlich travelled to Egypt and other countries in 1888 and 1889, in part to cure a case of tuberculosis which he had contracted in the laboratory.

Here he discovered in 1909 the first drug to be targeted against a specific pathogen: Salvarsan, a treatment for syphilis, which was at that time one of the most lethal and infectious diseases in Europe.

Among the foreign guest scientists working with Ehrlich at his institute were two Nobel Prize winners, Henry Hallett Dale and Paul Karrer.

Wilhelm II the German emperor, wrote in a telegram of condolence, "I, along with the entire civilized world, mourn the death of this meritorious researcher for his great service to medical science and suffering humanity; his life’s work ensures undying fame and the gratitude of both his contemporaries and posterity".

[8] In the early 1870s, Ehrlich's cousin Karl Weigert was the first person to stain bacteria with dyes and to introduce aniline pigments for histological studies and bacterial diagnostics.

He spent his eighth university semester in Freiburg im Breisgau investigating primarily the red dye dahlia (monophenylrosanilin), giving rise to his first publication.

He demonstrated the existence of nucleated red blood cells, which he subdivided into normoblasts, megaloblasts, microblasts and poikiloblasts; he had discovered the precursors of erythrocytes.

Ehrlich's great achievement, but also a source of problems during his further career, was that he had initiated a new field of study interrelating chemistry, biology and medicine.

When a student in Breslau, Ehrlich was given an opportunity by the pathologist Julius Friedrich Cohnheim to conduct extensive research and was also introduced to Robert Koch, who was at the time a district physician in Wollstein, Posen Province.

In 1887 Ehrlich became an unsalaried lecturer in internal medicine (Privatdozent für Innere Medizin) at Berlin University, and in 1890 took over the tuberculosis station at a public hospital in Berlin-Moabit at Koch's request.

[13] Emil Behring had worked at the Berlin Institute of Infectious Diseases until 1893 on developing an antiserum for treating diphtheria and tetanus but with inconsistent results.

This joint work was successful to the extent that Ehrlich was quickly able to increase the level of immunity of the laboratory animals based on his experience with mice.

Ehrlich resented what he considered as unfair treatment, and his relationship with Behring was thereafter problematic, a situation which later escalated over the issue of the valency[14] of tetanus serum.

[15] Since antiserums were an entirely new type of medicine whose quality was highly variable, a government system was established to guarantee their safety and effectiveness.

Ehrlich postulated that cell protoplasm contains structures which have chemical side chains (macromolecules) to which the toxin binds, affecting function.

For providing a theoretical basis for immunology as well as for his work on serum valency, Ehrlich was awarded the Nobel Prize for Physiology or Medicine in 1908 together with Élie Metchnikoff.

Metchnikoff, who had researched the cellular branch of immunity, Phagocytosis, at the Pasteur Institute had previously sharply attacked Ehrlich.

In this situation Althoff arranged a contact with Georg Speyer, a Jewish philanthropist and joint owner of the bank house Lazard Speyer-Ellissen.

Ehrlich had also received from the German Emperor Wilhelm II a personal request to devote all his energy to cancer research.

In 1885 Ehrlich's monograph "The Need of the Organism for Oxygen" (Das Sauerstoffbedürfnis des Organismus – Eine farbenanalytische Studie) appeared, which he also submitted as a habilitation thesis.

In Ehrlich's view, an added benefit was that methylene blue also stained the long appendages of nerve cells, the axons.

As a model for experimental therapy Ehrlich used a guinea pig disease trypanosoma and tested out various chemical substances on laboratory animals.

With the support of his assistant Sahachiro Hata Ehrlich discovered in 1909 that Compound 606, Arsphenamine, effectively combatted "spirillum" spirochaetes bacteria, one of whose subspecies causes syphilis.

After extensive clinical testing (all the research participants had the negative example of tuberculin in mind) the Hoechst company began to market the compound toward the end of 1910 under the name Salvarsan.

[21] Ehrlich's name is also borne by many schools and pharmacies, by the Paul-Ehrlich-Gesellschaft für Chemotherapie e. V. (PEG) in Frankfurt am Main, and the Paul-Ehrlich-Klinik in Bad Homburg vor der Höhe.