No visa is necessary for travelers from the U.S. You do need a passport.
There are no required immunizations to enter Costa Rica, however, some guidebooks recommend taking malaria pills to visit Costa Rica. The risk of contracting malaria in Costa Rica is minimal. And no Costa Ricans ever take malaria pills
Decisions about immunizations and anti-malarial medications should be made on a personal basis after consultation with your personal physician.
If you take prescription medication, be sure to have enough for the entire trip or have your doctor give you a spare prescription with a note suggesting an alternative medication if your first choice isn?t available.