What kind of inoculations do I have to take to Costa Rica?

Starting on July 30, 2007, the Costa Rican authorities will demand that all travelers coming from the following countries (Angola, Benin, Burkina Faso, Cameroon, Democratic Republic of the Congo, Gabon, Gambia, Guinea, Liberia, Nigeria, Sierra Leone, Sudan, Bolivia, Venezuela, Brazil, Peru, Colombia and Ecuador as well as the Republic of Guyana) have a yellow fever vaccination certificate.