Medicare is a federal health insurance program structured primarily to cover three distinct demographic groups: 1) Individuals aged 65 or older who are U.S. citizens or lawful permanent residents with sufficient work history; 2) Individuals under age 65 who receive Social Security Disability Insurance (SSDI) benefits for a qualifying period; and 3) Individuals of any age diagnosed with End-Stage Renal Disease (ESRD) requiring permanent kidney dialysis or a kidney transplant, or those diagnosed with Amyotrophic Lateral Sclerosis (ALS).