So, going back to your question about the argument against the public wearing masks, I think that there was a mixture of concern that the public would hoard and that it would create alarm. But, to this point, we are in alarming circumstances, and the risk is non-zero. Let’s say in the worst-case scenario that masks only offer fifty-per-cent protection. Well, fifty per cent is better than zero. And it seems to me, more and more, that the C.D.C. and the White House have come around to the idea that people should be wearing masks.

The other arguments I have read against them were about people scratching and touching their faces more, or about the difficulty of cleaning them, which introduces another risk.

If we ask the public to wear masks, whatever masks they are, we need a simple decontamination protocol. Because what we don’t want is people to wear masks, and feel safe, whereas, in fact, they are collecting respiratory droplets. So we need a simple decontamination protocol.

What about cleaning masks? The C.D.C. says that they should be routinely washed, and that a washing machine “should suffice.”

You should follow the C.D.C. recommendations, but I’m not aware of a lot of empirical data about whether washing degrades the fibres, and it would be very helpful if someone produced that data. I am agnostic, to be honest. If washing works, it works. I am totally fine with washing. I would follow the C.D.C. recommendations.

Just to emphasize: I do not have empirical data that simple masks work for the coronavirus. But we do know they work for other respiratory viruses. So what we are trying to do is extrapolate. There is no empirical evidence that I’m aware of that going to the grocery store with a mask will protect you more than anything else from the coronavirus. But with other viruses, including ones that behave similarly, it helps.

What sort of face coverings should we be using if we don’t have access yet to professionally made masks?

The quick answer is that I don’t know and we don’t know. I would vastly prefer that if we are going to ask the public to use masks, the C.D.C. or someone should give us very clear guidelines on what works and what doesn’t work. There is a study from Cambridge University in which they tested a number of household materials, ranging from cloth to linen, against the size of a typical respiratory droplet. The study suggested that T-shirts and pillowcases worked fine. Again, we are operating with little data. Surgical masks are the most effective in this study. Then a vacuum-cleaner bag and then a dish towel. Then a cotton-blend T-shirt. So that is probably a reasonable item to use in a double layer.

The problem is that if we have a free-for-all, and say you can make your own, then there is no control over it. Some people will make things that are really good, and some people will make things that are terrible. And we will think they are all identical and move right along. It would be very helpful if we could standardize this and have a decontamination protocol for public use.

Has the science changed on masks at all recently? Or has this always been what it has been?

There is more science now. There are more papers for SARS, which show that, in the hospital, a simple surgical mask, in conjunction with hand hygiene and social distancing, vastly decreased the infection rate. The problem with science is that it can’t catch up with what people are doing on the side. If everyone says they are going to use their own material, then science can’t catch up with it, because we don’t know what you are using. So there is more science, but the science has to go hand in hand with the social shifts. And it would seem to me that, rather than the President saying to cover your mouth with a scarf, having a scientific, centralized way to distribute it, produce it free of charge, and make everyone wear it, and have a decontamination protocol would be crucial.

Make everyone wear it, or suggest that they do?

I would suggest it, and if it really makes a dent, then put out a public-service announcement saying everyone should wear it. You can’t force people to adopt behaviors in this country, although you can make it very uncomfortable for people who show up without masking because of social pressures. But I don’t think it should be compulsory. I do think that, given the seriousness of what is going on, given the paralysis in the economy, and the desire for people to get back to work, I think there should be some sense of social responsibility, and if it really decreases the chance of infecting others and yourself, and if people agree universally to wear it, I think we could potentially convince the vast majority of the people to wear a mask.

Does this contradict what you said earlier about not wanting to shame people who don’t wear it? Could we get to a point where we should be shaming people who don’t?

I think shame is not the best mechanism for getting people to wear it. I think the better mechanism is for everyone to understand that there is a sense of social responsibility that they have, and adopting that social responsibility is part of what we do as a country to prevent economic collapse. If we don’t get back to work by June or July—hopefully, there will be medicines by then. But there will not be vaccines, and if we want to get back to work by June or July, I strongly suggest that we find mechanisms by which we can get back to work with masks.

Lots of evidence has shown that masked countries are doing better than unmasked ones in terms of spread. Do you attribute that to the masks?

It’s very hard, because there is no trial and we have to believe the evidence at face value. There was a meme going around Twitter showing that countries with masks were doing better, but then someone superimposed that same meme and instead of masks they wrote bubble tea and no bubble. It is suggestive, but it is an unknowable, because no one did the experiment, and the scientist in me has to say that it’s an unknowable because there is no direct evidence that the countries wearing masks were more successful in decreasing the rise in infections.

And, in South Korea, say, it could be about something else, like testing.

It could be about testing. Yes.

Is mask wearing something that would generally be healthy and cut down on other diseases and flu deaths? Is this a societal change we should be making or thinking about apart from the coronavirus?

We know, again, from a vast amount of scientific evidence that viruses that are secreted through respiratory secretions and cough droplets, including influenza and SARS, are decreased in their exposure when people wear masks. So, under conditions when the infections are spreading fast, the evidence suggests that mask wearing really decreases the chance of infecting others and getting infected yourself. And the return on the so-called R0 of the virus may be quite large. So I think in conditions such as now, when there is exponential spread of the virus, I think that would be the right thing to do.

A Guide to the Coronavirus