Modern medical research also indicates that the common tropical disease known as dengue, or “breakbone fever,” can also have the resulting effects of an immunity to yellow fever. But no human being ever achieved an immunity to malaria; there was no such thing as a natural immunity to yellow fever, and if many blacks had been made immune to yellow fever before reaching Panama, there were vastly more blacks at work than whites, so the number of nonimmune blacks on the Isthmus was always quite large. Black laborers died of both malaria and yellow fever and no less miserably than the whites.
