Medicare Part A covers hospitalization benefits. It covers all costs for most hospital stays, except there is a deductible of about $1260 in 2015.
All out-patient benefits are provided by Medicare Part B. These include doctor visits for illness as well as wellness care, testing etc.. Note that you must pay for Medicare part B. The cost is generally about $105-110/mo, unless you are in a higher income bracket. There is a deductible for Part B benefits also. It is $137 in 2015.
Medicare Part B covers about 80% of the costs of out-patient care if the service is provided by a practitioner who/which accepts Medicare. A Medicare Supplement could pay much or all of the deductibles and co-pays under Medicare. You must continue to pay the Part B premium.
If you choose to be covered by a Medicare Advantage Plan, INSTEAD of Medicare, you must continue to pay the Medicare part B premium.