Yellow fever is found in a belt of countries concentrated around the equator in Africa and South America. It is part of the lovely group of hemorrhagic diseases and has been on an upswing worldwide since the 1980s.
Getting a yellow fever vaccine isn't as simple as a Hep A vaccine or other adult immuniziations. Here in the RTP area, Duke Travel Clinic is one of the few places that the vaccine is given. (A registry of providers can be found on the CDC website.)
Proof of yellow fever vaccination is required for entry for many countries, with the proof given by an International Vaccination Certificate (IVC, a picture is shown at the bottom of this link to the CDC). A sticker from the yellow fever vaccine must be placed on this form and the health care provider has to sign it. If you don't get this form when you leave your doctor's office, you might as well not have gotten the vaccine in the eyes of those countries that require documentation.
The importance of the vaccine is underscored by information on the CDC website on the outcomes of unvaccinated travellers. While only nine people came down with yellow fever, eight of those nine didn't make it.