Costa Rica is a very healthy country. Generally it is a clean, safe destination with few risk factors given normal precautions. The water is generally drinkable, but bottled water is available widely. Food preparation is usually very clean.
Malaria exists in some of the low lying areas where mosquitoes are prevalent, but not in San Gerardo. You will need to use insect repellant just to protect against excessive bites. However, there are poisonous snakes, and hiking involves other risks such as falls, sunstroke, dehydration, cold at night, and disorientation: so caution must be exercised in this remote region. Bring a First Aid kit!
A doctor visits San Gerardo once every two weeks to see patients at the Health Centre (in the office next to the salon), otherwise medical attention is available in San Isidro via one of the many local doctors, or (with a wait) at the San Isidro General Hospital.
Health insurance is a must for travelers as costs for non-residents are high. The Red Cross (Cruz Roja Costarriciense) operates the ambulance system.