SAN FRANCISCO — The rumor unsettled Deborah Santamaria.

A fellow janitor at 555 California Street, a 52-story office tower in San Francisco’s financial district, told her he heard that a floor of the building was being closed because a worker had contracted the novel coronavirus. At 63, Ms. Santamaria counted herself among those most vulnerable to a virus that had killed thousands worldwide and was rapidly spreading across the United States.

Her supervisor at Able Services, the contractor that employs her, reassured her that nothing was wrong, she said.

It was not until five days later that a news article appeared saying that Wells Fargo had temporarily evacuated its offices in the building after an employee had tested positive for the coronavirus.

The bank had notified building management, which alerted the cleaning contractor. But according to the employees and their union representatives, no one had told the janitors.

“I felt as if I didn’t matter,” said Ms. Santamaria, who earns $22 an hour.

While many Americans are fleeing their offices to avoid any contact with the coronavirus, low-wage janitors are sometimes being asked to do the opposite. Although millions of Californians have been ordered to shelter in place, janitors are still being asked to go into offices to battle the invisible germs that threaten public health, even as those germs, and the new, powerful cleaning solutions they are being asked to use, may endanger their own health.