The popular Pacific coast community of Dominical has seen a doubling of dengue cases from 20 to 40.
The mosquito-borne disease spread despite four spraying campaigns by health workers, said resident Steve Fergus.
However, he was quick to praise these efforts and said that health workers have been going from house to house seeking those who are ill.
Health workers also have been spraying heavily in the Jacó area further north up the coast where at least 11 cases of zika have been detected.
The same mosquito carries both diseases.
The increase in dengue cases was unexpected because of the abnormally dry season caused by the climate conditions in the Pacific. Now that the rainy season has arrived, more cases are expected among those who do not take preventative measures.
Some tourism operations already are reporting cancellations due to zika fears.