The U.S. State Department
states that

" Uninsured travelers who require medical care overseas
may face extreme difficulties."

The following is quoted from the Website of the U.S. Department of State– Bureau of Consular Affairs– Medical Information for Americans Traveling Abroad. They advise all international travelers that:

U.S. Medicare and Medicaid programs do not provide payment for medical services outside the United States.

The social security medicare program does not provide coverage for hospital or medical costs outside the U.S.A.

Private U.S. medical insurance is not always valid outside the United States.

Doctors and hospitals often expect immediate cash payment for health services. Please check with your own insurance company to confirm whether your policy applies overseas, including provision for medical evacuation. Please ascertain whether payment will be made to the overseas hospital or doctor, or whether you will be reimbursed later for expenses that you incur.

If an American citizen becomes seriously ill or injured abroad, a U. S. consular officer can assist in locating appropriate medical services and informing family or friends. If necessary, a consular officer can also assist in the transfer of funds from the United States. However, payment of hospital and other expenses is the responsibility of the traveler.

Before going abroad, learn what medical services your health insurance will cover overseas. If your health insurance policy provides coverage outside the United States, remember to carry both your insurance policy identity card as proof of such insurance and a claim form. Although many private health insurance companies will pay "customary and reasonable" hospital costs abroad, very few will pay for your medical evacuation back to the United States. Medical evacuation can easily cost $10,000 and up, depending on your location and medical condition.