Went to Chiapas, Mexico, and adjacent Guatemala some years ago, and websites said malaria wasn't a problem, but when we got there, the local newspapers had articles about malaria in the region. We were collecting orchids, and thus were out in the bush. A family we were staying with steered us to a doctor for advice. It seems repellent was sufficient. Also most of our time was spent at temperate altitudes (orchids do not like hot, steamy jungles), so we didn't see any mosquitos.
Deet takes care of the mosquito problem.

A good website about disease in Costa Rica:
www.therealcostarica.com/health_education_costa_rica/disease_costa_rica.html

Costa Rica should give you no problems- Many tourists and little illness.