Do I Need SHOTS? What about Malaria/Dengue?
From Costa Rica Travel Guide: Vacation and Travel tips
So, I'll say it: "If you're sticking to the more touristy destinations when visting Costa Rica, the likely hood of catching any 'tropical' disease is fairly low, again, possible but not probable."
Now, if you're venturing OFF the beaten path, volunteering or doing work in remote or impoverished areas of Costa Rica, staying here for extended periods of time or living in areas more susceptible to outbreaks of Dengue or Malaria, such as Limon, then yeah, TOO MUCH information is a GOOD thing and you should be doing your HOMEWORK.
Here's an overview: Travelers Diarrhea is the most common ailment when traveling just about ANYWHERE. Drink bottled water, make sure vegetables and fruits are washed, peeled and cooked. Travel Books on Costa Rica will usually tell you not to use ice cubes or buy your food from street vendors, we do BOTH, never had a problem. Not telling you to... just sayin.
But in Costa Rica, most travelers are concerned with contacting Dengue or Malaria which are both spread through mosquitoes. Both incidents are low, but if you're concerned or some of the above variables apply to your situation, then the answer is easy, mosquito repellent should be frequently applied, proper attire such as long-sleeves and pants etc, mosquito netting at night and/or coils and in the case of Malaria, maybe the Malaria pill is the perfect option.