The company may be under criminal investigation for alleged irregularities in its Medicare business—but it’s been touched by the Warren Buffett effect. Shares in the troubled private health insurer UnitedHealth Group (NYSE: UNH) are currently seeing their best trading day in months.

UnitedHealth Group's public reputation is still reeling, yet its battered stock has been something that more and more investors can't seem to resist. Why it matters: Recent interest in its stock, from everyday investors to Wall Street's titans,