There were several reported cases of malaria in the Guanacaste last year, but I would not worry about this. The anti-malarias can cause all kinds of side effects and the risk of contracting the disease are so low that I would never bother taking it, but that is just me. You will have to make your own informed decisions about this.
Make sure not to take the wrong anti-malaria, which a friend of mine did. They were prescribed Cloroquine, which is effective in South America, not Central America, and they ended up at the Embassy telling tales of how they were being hunted by men with machetes. Little did they know that they were hallucinating (a side effect) and the men were merely friendly farmers (who just happen to carry machetes, everywhere, because its the most important tool if you live in the jungle).
Dengue on the other hand is much more common and is not fun. I was diagnosed with a case last September. The symptoms are similar to the flu, aches, pains and high fever. It was by no means enjoyable especially when the accompanying hemorrhagic rash appeared, that was when I thought I was going to die. Fortunately, I did not. All this could have been avoided if I had worn insect repellent with at least 25% Deet, religiously, but I was too lazy and paid the price for it. However, I will be wearing bug spray this season, you can count on it.
