“We arrive at this moment with the overleveraged corporate sector about to face the prospect that new-issue bond markets may seize up, as they did last week, and that even seemingly sound companies will find credit expensive or difficult to obtain,” Mr. Minerd wrote in a note on Sunday.

Leveraged loans, which banks and other financial institutions extend to already indebted companies, have also ballooned. Outstanding leveraged loans now total about $1.2 trillion, roughly twice pre-crisis levels. The loans are often funded by investment vehicles that bundle, slice and sell them off to investors — and because banks do not end up holding many of them on their balance sheets, regulators have limited ability to oversee them.

To put the size of those vulnerable debt and loan piles in perspective, Americans had $1.3 trillion in outstanding subprime mortgages headed into the 2008 crisis, based on Organization for Economic Cooperation and Development estimates. When those debts went bad, it helped knock the financial system to its knees.

Investors are already spooked about risky credit in a time of coronavirus. Indexes that track high-yield debt and leveraged loans have had record outflows over the past two weeks, as people take their money and run, seeking safer investments.

“We are concerned about potential cracks in the U.S. credit cycle in an environment of dwindling corporate cash flows, which could lead to a sharp tightening of financial conditions that feeds back into the real economy,” Joachim Fels, global economic adviser at Pimco, wrote in a research note Sunday.

While banks have limited direct exposures to the riskiest loans — which could keep any messiness in that sector from turning into a full-blown financial meltdown — they and the broader economy would still be affected if things soured quickly.

“In the event of corporate debt distress, firms may reduce investment and cut payrolls to continue servicing their debt,” according to one 2019 Federal Deposit Insurance Corporation report. That could make it harder for households and businesses — bank customers — to pay back their own debt, and “banks could incur losses.”