In another tweet, Dr. Adams said the best way to protect against the virus is to wash hands regularly, and for those who are feeling ill to stay home.

Health officials around the world have been imploring the public to stop buying masks if they are healthy or not caring for someone who is ill. Medical professionals need a large supply of the masks because they are in direct contact with infected patients and must change their masks repeatedly.

“There are severe strains on protective equipment around the world,” said Dr. Michael J. Ryan, executive director of the health emergency program at the World Health Organization, during a briefing on Friday. “Our primary concern is to ensure that our front line health workers are protected and that they have the equipment they need to do their jobs.”

Dr. Ryan said masks primarily prevent a person from giving the disease to someone else.

“There are limits to how a mask can protect you from being infected,” he said. “The most important thing everyone can do is wash your hands, keep your hands away from your face and observe very precise hygiene.”