On a mid-March midnight shift, one of Kristy Granados’s co-workers coughed and said he felt nauseated.

As he took a break, workers at the Amazon.com Inc. warehouse in Charlotte, N.C., shared worries that they could catch the coronavirus, Ms. Granados said. Most didn’t have masks or supplies to disinfect workstations, even as cleaning products streamed past them on conveyor belts. On some recent days, about half the workers hadn’t even shown up, she said.

Her...