Yellow fever is a disease that is caused by a virus, and is common in jungle environments in Central America, the Caribbean, Africa, and Southeast Asia. It is transmitted through bites from infected mosquitoes. Once aquired, most patients survive, although a significant portion (39%) die. Either way, the victim is never infected again. There is currently no treatment for it, so doctors can only try to treat the symptoms. 
During the construction of the Panama Canal, yellow fever came and went in waves, paralyzing the entire community. Other than malaria and consumption of unripe bananas, it was one of the worst killers of the people in Panama. Although for years it had been a threat to the natives, canal workers were especially vulnerable.
Symptoms include fits of shivering, high fever, and an unquenchable thirst; similar to malaria. However yellow fever also causes severe vomiting of a distinctively dark color, which is known among the natives as vomito.