The Panama Canal required solving a disease that killed 20,000 workers

France's first attempt to build a canal across Panama in the 1880s killed an estimated 20,000 workers — not from construction accidents but from yellow fever and malaria, then thought to be caused by 'bad air'. When the United States took over in 1904, chief sanitation officer William Gorgas drained standing water, installed window screens, and fumigated buildings across the Canal Zone — based on the new understanding that mosquitoes, not air, spread the disease. Yellow fever was eliminated from Panama within two years. Without this public health breakthrough, the canal could not have been built.

Source: McCullough, The Path Between the Seas (1977)