That's a double edged sword. I love your spirit, but I wonder if that is taking an unneccessary risk. Especially if you're gonna go backcountry. The roads are notoriously bad, weather can be unpredictable, and the flora and fauna is extreme and unusual. Not too mention the bugs !!!!
God forbid if your infant got really really sick, would you second guess your decision. My mother-in-law and father-in-law went there for an anniversary trip 5 years ago, and my father-in-law got sooo sick he had to take an early retirement. My mother-in-law didn't get anything. They still don't know what he got, but he was sick for several years. And it was soooo bad he couldn't continue to work. If an infant got something like that it could be fatal.
We're having our first baby next month, and we travel alot. We've been to Costa Rica twice and we were married there a year ago. We've talked about traveling with our child, and we decided to wait until she is older. That way she will remember it, plus she will be less suseptible to illness.
Remember, Costa Rica is still mostly wild rainforest.