Berkshire Hathaway recently filed its second-quarter 13F form, revealing its stock holdings at the end of June.

At Berkshire Hathaway’s annual meeting earlier this year, Warren Buffett showed just how much of a fan he is of Apple’s CEO, Tim Cook. “Tim Cook has made Berkshire a lot more than I have made Berkshire,” Buffett said to laughter in the room. It was funny, but considering that Apple is Berkshire’s largest holding, I don’t think it was hyperbole.

The investment comes as UnitedHealth faces soaring medical costs, federal investigations, the fallout of the killing of a top executive and a cyberattack last year.

Buffett owned UnitedHealth shares before selling his stake in 2010, though his Berkshire Hathaway recently acquired shares.

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Leveraged positions in the stock market jumped 18% in one month and topped $1 trillion for the first time. Increasing use of debt to invest can tell us a lot about investors' attitudes. And while this has always been the case,