"It's very, very frustrating from the perspective that you don't know how much more they know that they're not telling you" – Standard & Poor's Shannan Murphy about Millennium Health.

Millennium Health – biggest drug-testing lab in the US and biggest recipient of Medicare drug-testing payments, which account for one-third of its revenues – is Exhibit A of how a credit bubble allows companies and banks to put yield-desperate investors, blinded by a zero-interest-rate policy, through the wringer.

The San Diego-based company is owned by its executives and private-equity firm TA Associates. It issued a junk-rated loan of $1.775 billion due in 2021 that JPMorgan syndicated in April 2014. The pieces were eagerly bought up by institutional investors, among them Oppenheimer Funds, Fidelity Investments, and Franklin Resources. They'd stuff part of it into mutual funds owned by retail investors.