Mild infection may go unnoticed. This disease mimics malaria, hepatitis, typhoid, dengue fever, and leptospirosis. Thousands of cases are reported each year throughout the endemic zone - many more go unreported. Tourists occasionally contract yellow fever. Some countries require it only if entering from a country within the yellow fever endemic zone, others require proof even if entering from the U.S. Countries that require proof of vaccination are not only those within the endemic zone but many others where the disease could potentially begin again.
Vaccination is extremely effective.
If in an active yellow fever area, extreme precautions against mosquito bites should be taken.