Buffett sparks UnitedHealth's biggest stock surge in 17 years

1. Warren Buffett invested $1.6 billion in UnitedHealth, boosting share prices. 2. UnitedHealth shares are experiencing their best one-day gain in years. 3. The stock remains down over 50% in the past year despite recent gains. 4. UnitedHealth faces scrutiny from a Department of Justice probe. 5. The murder of executive Brian Thompson raised concerns over the healthcare system.