If you see a snake during your trek, keep a distance. You do not want to be bitten in an area with very bad communications.
Some of the snakes' poisons can kill you in less than four hours, and if you are inside the park it will take you at least five hours to get to the doctor. If you hire a guide, make sure he knows the local fauna.
In the picture you can see one of the poisonous snakes in the Corcovado Park, the Eye Lash Viper.