Born in South Africa, Theiler moved to the US in 1922 and became known for his research on yellow fever, encephalomyelitis, and other tropical viruses. At a time when yellow fever outbreaks were plague-like—one epidemic alone wiped out 20,000 people—his team proved that the disease is caused by a virus, not bacteria. Theiler’s development of a safe and effective vaccine brought yellow fever under control, earning him the 1951 Nobel Prize in medicine. On what did he initially test his vaccine? Discuss