In recent weeks, a new cadre of gatekeepers armed with thermometer guns has appeared at the entrances of hospitals, office buildings and manufacturing plants to screen out feverish individuals who may carry the coronavirus.

Employees at some companies must report their temperature on apps to get clearance to come in. And when indoor dining resumes at restaurants in New York City later this month, temperature checks will be done at the door.

Since the beginning of the pandemic, the practice of checking for fever has become more and more commonplace, causing a surge in sales of infrared contact-free thermometers and body temperature scanners even as the scientific evidence indicating they are of little value has solidified.

WASHINGTON — The coronavirus pandemic remained a distant threat to many Americans on Jan. 24 when Douglas C. Bryant, the president and chief executive officer of Quidel Corporation, got a call from an official at the Food and Drug Administration.

If the government helped, the official asked, would Quidel make tests to detect coronavirus infections?

Mr. Bryant decided to go for it, even though some experts at the time were predicting the United States would quickly tame any outbreak.