Wikipedia notes: Colonel William C. Gorgas had been appointed chief sanitation officer of the canal construction project in 1904. Gorgas implemented a range of measures to minimize the spread of deadly diseases, particularly yellow fever and malaria which had recently been shown to be mosquito-borne following the work of Dr. Carlos Finlay and Dr. Walter Reed. There was investment in extensive sanitation projects, including city water systems, fumigation of buildings, spraying of insect-breeding areas with oil and larvicide, installation of mosquito netting and window screens, and elimination of stagnant water. Despite opposition from the Commission (one member said his ideas were barmy), Gorgas persisted and when Stevens arrived, he threw his weight behind the project. After two years of extensive work, the mosquito-spread diseases were nearly eliminated. Nevertheless, even with all this effort, about 5,600 workers died of disease and accidents during the U.S. construction phase of the canal.