Face masks are emerging as one of the most powerful weapons to fight the novel coronavirus, with growing evidence that facial coverings help prevent transmission—even if an infected wearer is in close contact with others. Robert Redfield, director of the Centers for Disease Control and Prevention, said he believes the pandemic could be brought under control over the next four to eight weeks if “we could get everybody to wear a mask right now.” His comments, made Tuesday with the Journal of the American Medical Association, followed an editorial he and others wrote there emphasizing “ample evidence” of asymptomatic spread...