Having been introduced to the idea of wearing masks to protect themselves and others, some Americans are now considering a behavior scarcely seen in the United States but long a fixture in other cultures: routinely wearing a mask when displaying symptoms of a common cold or the flu, even in a future in which Covid-19 isn’t a primary concern. “I will still feel a responsibility to protect others from my illness when I have a cold or bronchitis or something along those lines,” said Gwydion Suilebhan, a writer and arts administrator in Washington who said he also plans to continue wearing masks in situations like flying on airplanes. “It’s a responsible part of being a human in a civil society to care for the people around you.” Such routine use of masks has been common for decades in other countries, primarily in East Asia, as protection against allergies or pollution, or as a common courtesy to protect nearby people.