What medical discovery is depicted in Dr. Ehrlich's Magic Bullet?

The film Dr. Ehrlich's Magic Bullet (1940), directed by William Dieterle, depicts the scientific work of the German scientist Paul Ehrlich and his discovery of a cure for syphilis.1

Ehrlich and his colleague Sahachiro Hata developed arsphenamine—commercially known as Salvarsan—which was a synthetic preparation containing arsenic. This treatment was lethal to the microorganism responsible for syphilis and successfully inaugurated the chemotherapeutic era, revolutionizing the control of infectious diseases. The term "magic bullet" refers to a substance or therapy capable of destroying pathogens without causing deleterious side effects.23