Abstract
Summary A reorientation of historical source material is necessary before conclusions can be drawn concerning the antiquity of sylvan yellow fever. During the building of the Panama Railroad (1850–1855) yellow fever was singularly absent from the mainland, but during the operation of the first French Canal Company (1881–1889) it took a heavy toll of human lives. After 1889 it remained endemic in Panama City until its eradication by Gorgas in 1905. From 1906–1919 only imported cases developed in the Canal Zone and from 1920 to 1948 no further cases were encountered. Surveys of the human population indicate that sylvan yellow fever has been enzoötic in eastern Panama since 1929. In November and December 1948 a wave of sylvan yellow fever began moving westward from this enzoötic center, following the Atlantic rain forest, and reached northern Costa Rica in July 1951. Early in October 1951 an offshoot of this wave had crossed the Cordillera of southern Costa Rica to the Pacific side and has moved eastward again, re-entering western Panama, but now on the heretofore uninvolved Pacific watershed, west of the Canal. The main front of the wave is expected to continue moving into Nicaragua.