“It was the only time that I wished I’d had a mask,” he said.

So do masks really work? The answer is yes and no. Dr. Tierno said he had seen people wearing surgical masks on the subway and it was “like having no mask at all.” Air can seep in through the gaps. A cloth mask, too, provides little protection. Sometimes mask wearers cover only their mouths, leaving noses exposed. “For most people, a mask is not necessary,” he said.

In February, the surgeon general urged the public to stop buying masks, warning that it won’t help against the spread of the coronavirus but will take away important resources from health care professionals.

“Seriously people — STOP BUYING MASKS!” the surgeon general, Jerome M. Adams, said on Twitter.

When one is needed — mostly in a place where a lot of illnesses have been reported — people should wear an N95 respirator, a heavy-duty mask fitted to the face that filters out 95 percent of smaller air particles. But, Dr. Tierno warned, “it is a very tough mask to breathe through.”

Please! Wash your hands.

This seems so simple that Trevor Noah recently made a joke about it on “The Daily Show.” Health professionals say washing hands with soap and water is the most effective line of defense against colds, flu and other illnesses.

Just think about where your hands have been in the past 24 hours. Now, think about all the hands that have touched airplane tray tables and seatbelt buckles. If that doesn’t give you pause, consider whether you bite your nails, touch your face or rub your eyes. “The 10 dirtiest things are your fingers,” Dr. Tierno said.