According to a recent 13F filing, Berkshire Hathaway now owns 5 million shares of UnitedHealth Group. Shares of health insurer UnitedHealth Group (UNH -1.38%) received a boost last week after investors learned that Warren Buffett's company, Berkshire Hathaway (BRK.A 1.28%) (BRK.B 1.35%), has invested in it. It has been a tumultuous year for UnitedHealth, which has seen its shares hit multi-year lows due to underwhelming quarterly results and a barrage of bad news. Entering trading this week, UnitedHealth st ...