Paul Ehrlich

Paul Ehrlich (14 March 1854 – 20 August 1915) was a German doctor of Jewish descent. He won the 1908 Nobel Prize in Physiology or Medicine, with Ilya Mechnikov, for finding out how immunity fights disease.
Ehrlich directed a research program which discovered the first treatment for syphilis which worked. It was a compound of arsenic which went by the trade name of "Salvarsan". He studied trypanosomiasis and other protozoal diseases. He produced trypan red, which was (as his Japanese assistant Shiga showed) effective against trypanosomes.
He invented a staining system for bacteria (before Gram staining). He did a complete survey of all the cell staining techniques then known. He showed how to tell the difference between different kinds of white blood cells, and discovered mast cells. He discovered that methylene blue, which stains bacteria and cell nuclei, could put the malarial parasite, "Plasmodium", into recession (the fever drops).
He also made a decisive contribution to an antiserum against diphtheria, and developed a method for standardizing therapeutic serums. His co-worker got a sole Nobel Prize for the joint work on diphtheria.