In order to get into Costa Rica, yellow fever vaccine is mandatory:
If you travel to Costa Rica from South America and/or sub-Saharan Africa, will need to file the international certificate of YELLOW FEVER VACCINATION . The countries concerned are: Angola, Benin, Burkina Faso, Cameroon, Democratic Republic of Congo, Gabon, Gambia, Ghana, Guinea, Liberia, Nigeria, Sierra Leone, Sudan, Bolivia, Brazil, Colombia, Ecuador, Peru, French Guyana and Venezuela . You can travel to Costa Rica ten days after the application of the vaccine.There are some exceptions
This is referential information only, we recommend to consult with your travel agent.
Information taken from: http://www.costarica-embassy.org/?q=node/93