President Barack Obama's health overhaul encourages prevention by requiring most insurance plans to pay for preventive care. On the plus side, more than 22 million Medicare patients and many more Americans with private insurance have received one or more free covered preventive services this year. From cancer screenings to flu shots, many services no longer cost patients money. But there are confusing exceptions. Colonoscopies, for example, can go from free to pricey while the patient is under anesthesia.