Although scientists cannot experiment on human beings deliberately,
some wonder if the world is now carrying out a natural experiment that tests the value of mask-wearing.
In many East Asian countries it was common practice to sport masks, even before covid-19, to protect against respiratory diseases and pollution.
A lot of people in these places therefore took immediately to wearing masks when the epidemic started.
Countries that adopted masks early on did not, by and large, shut their economies down.
Yet they suppressed the disease more effectively than those that locked down but did not wear masks.
In the West nobody normally wears a mask, though the practice is spreading.
Universal masking started in the Czech Republic after Petr Ludwig, a Czech YouTube star,
posted a video on March 14th recommending the practice, and it went viral. Other social-media influencers posted pictures of themselves wearing masks.
"Mask trees", where people would bang homemade masks for others to use, sprang up on street comers.
By March 19th masks were mandatory in the country. Slovakia and Slovenia followed swiftly.
In Britain the government advises people to wear masks, but to little effect. On the London Underground around a third of travellers do so.
On the Paris metro where people risk a 135 euros fine if they fail to cover their faces, everybody does.
In America the Centres for Disease Control and Prevention, which previously recommended mask-wearing only for health workers, changed its mind in early April.
It now recommends that everybody should wear them in places where it is hard for people to stay far enough apart.
Several states have passed regulations along those lines, as has New York City. But, as Governor Burgum noted, the rows go on.