Because Costa Rican authorities require a yellow-fever vaccine certification from travelers of the following countries: Angola, Benin, Burkina Faso, Cameroon, Democratic Republic of Congo, Gabon, Gambia, Guinea, Liberia, Nigeria, Sierra Leon, Sudan, Bolivia, Venezuela, Brazil, Peru, Colombia, Ecuador and Guyana, even if you were only transiting (a stop-over) through any one of these countries. Please contact the Costa Rican consulate in your country to ask how many days prior to arriving into Costa Rica do they require that you have the yellow fever vaccination. If so, you must travel with your yellow fever vaccination card to show the Costa Rican officials at Santa Maria International in San José. If you are over 60 years of age, although you may be exempt from the vaccination, you must provide a doctor to sign off on a RX pad saying you are not advised to take the vaccination. For other exemptions, please read:
http://www.costarica-embassy.org/index.php?q=node/97