There are two known cycles of yellow fever transmission, the jungle and urban cycles
The jungle (or sylvan) cycle
The jungle cycle which was discovered in the 1930s, involves transmission of the disease between monkeys by various mosquitoes. In the Americas, mosquitoes of the genus Haemagogus are the primary vectors. In Africa, the jungle cycle is maintained chiefly by the Aedes mosquitoes. Transmission from monkey to man is accidental and is the result of human penetration into infected areas.

The urban cycle
The urban cycle involves person to person transmission by the Aedes aegypti which has now been virtually wiped out in the Americas owing to the efforts to eradicate the vector. The urban yellow fever is often traced to persons who had been infected in forested areas and had brought the disease into the city, and less frequently, by an infected monkey which has strayed into human settlements and is bitten by an Aedes mosquito.

Incubation Period of Yellow Fever
About three to six days (six days recognized under International Health Regulations).