In the United States, Medicare is a federal social insurance program that provides health insurance to people over the age of 65, individuals who become totally and permanently disabled, end stage renal disease (ESRD) patients, and people with ALS. Recent research has found that the health trends of previously uninsured adults, especially those with chronic health problems, improves once they enter the Medicare program. Traditional Medicare requires considerable cost-sharing, but ninety percent of Medicare enrollees have some kind of supplemental insurance—either employer-sponsored or retiree coverage, Medicaid, or a private Medigap plan—that covers some or all of their cost-sharing. With supplemental insurance, Medicare ensures that its enrollees have predictable, affordable health care costs regardless of unforeseen illness or injury.
Private health insurance may be purchased on a group basis (e.g., by a firm to cover its employees) or purchased by individual consumers. Most Americans with private health insurance receive it through an employer-sponsored program. According to the United States Census Bureau, some 60% of Americans are covered through an employer, while about 9% purchase health insurance directly. Private insurance was billed for 12.2 million inpatient hospital stays in 2011, incurring approximately 29% ($112.5 billion) of the total aggregate inpatient hospital costs in the United States.
Health insurance in the United States is any program that helps pay for medical expenses, whether through privately purchased insurance, social insurance, or a social welfare program funded by the government. Synonyms for this usage include "health coverage", "health care coverage", and "health benefits". In a more technical sense, the term "health insurance" is used to describe any form of insurance providing protection against the costs of medical services. This usage includes private insurance and social insurance programs such as Medicare, which pools resources and spreads the financial risk associated with major medical expenses across the entire population to protect everyone, as well as social welfare programs like Medicaid and the Children's Health Insurance Program, which both provide assistance to people who cannot afford health coverage.