The chief executive of Fox News, Suzanne Scott, reacted swiftly to the threat of the coronavirus in late February: She ordered the bright, open new offices disinfected, installed hand sanitizer stations around the office and boldly canceled the company’s major ad sales event.

But her influence doesn’t extend to the most important part of Fox News: its programming in prime time.

There, for two crucial weeks in late February and early March, powerful Fox hosts talked about the “real” story of the coronavirus: It was a Democratic- and media-led plot against President Donald J. Trump. Hosts and guests, speaking to Fox’s predominantly elderly audience, repeatedly played down the threat of what would soon become a deadly pandemic.

The person who could have stopped the flow of misinformation was Ms. Scott’s boss, Lachlan Murdoch, the chief executive of the Fox Corporation. But he wasn’t paying much attention.