In the early 20th century, yellow fever was a major public health threat, particularly in tropical regions of Africa and the Americas, leading to countless outbreaks and significant mortality rates. The urgent need for an effective vaccine prompted researchers like Max Theiler to engage in intensive studies. Theiler, who had previously worked on vaccine development during World War I, joined the Rockefeller Foundation in New York City, where he had access to a plethora of resources and scientific minds. After extensive experimentation, Theiler was able to develop a live attenuated vaccine against yellow fever, using a strain of the virus that had been passaged through mouse brains, thus reducing its virulence while preserving its immunogenic properties.The achievement was groundbreaking not only because it directly addressed a critical public health issue but also because it exemplified the application of modern virology techniques. The vaccine Theiler developed was first used extensively in 1938, and its deployment drastically changed the course of yellow fever management, especially in endemic regions. The results were incredibly promising, leading to large-scale vaccinations that significantly reduced the incidence of the disease. Theiler’s work was recognized globally, and he was awarded the Nobel Prize in Physiology or Medicine in 1951 for his contributions to medicine.