When you or I leave the house, it is recommended we wear a mask for a fairly simple reason: If we have been infected with the coronavirus but aren’t aware of it, the mask can catch exhaled droplets that might contain the virus and infect people with whom we come into contact. It’s not a perfect preventive mechanism, but it’s an effective one. Think of it like using your turn signal. The benefit is largely in making the whole system safer more than just protecting you.

This is why the White House and President Trump argue he doesn’t need to wear a mask.

Asked why the president has so often been reticent to wear a mask, White House press secretary Kayleigh McEnany argued Tuesday it was because of how frequently he was tested for the virus.

“As I’ve made clear from this podium, the president is the most tested man in America,” she said. “He’s tested more than anyone, multiple times a day. And we believe that he’s acting appropriately.”

In other words, since the White House is confident he's not infected and since the point is to prevent someone from infecting others, no mask needed.

Speaking at a news briefing later in the day, Trump claimed he nonetheless wore a mask as recommended.

“If you’re close to each other, if you’re in a group, I would put it on,” he said. “When I’m in a group — if I’m in an elevator and there are other people with me, including, like, security people, it’s not their fault. They have to be in the elevator. I want to protect them also. I put on a mask.”

This assertion is certainly questionable. He had been asked about a video that surfaced showing him in the Trump hotel in Washington not wearing a mask. (The city is investigating whether local mask-wearing mandates were violated.) In the past, Trump had explicitly rejected the idea of wearing a mask, at one point suggesting he couldn’t see himself meeting foreign leaders while wearing a face covering.

Regardless, the White House has been consistent. Trump generally doesn’t need a mask, because he’s tested all the time, as are the people around him.

That, however, is not the protection it seems, for him or for those he comes into contact with.

I spoke by phone with Lisa Maragakis, an infectious-disease physician at the Johns Hopkins University School of Medicine and the senior director of infection prevention within the Johns Hopkins Health System. She said that, among other things, a negative coronavirus test is by itself no guarantee that someone isn’t contagious to others.

“We have certainly seen individuals that have clinical symptoms, for instance, and we know that they look like they have covid-19,” the disease the virus causes, she said, “and yet repeated tests are negative. And we do believe that that person has the disease and is capable of passing on the infection.”

“In those instances, we have been able to later recover the virus or to recover the virus from other means of testing,” she added, “like getting a sample from lower in their lungs.”

The example she gave was explicitly setting aside questions about the reliability of the tests themselves. The White House uses a rapid testing system created by Abbott Laboratories to screen those who come in contact with the president. In May, the Food and Drug Administration released a public notice focused on questions about that system’s reliability. One study conducted by researchers at New York University this year found a high rate of false negatives when using the testing system, with nearly half of individuals positively identified as having the virus by a competing test system getting a negative result in the Abbott test.

There isn’t a hard and fast timeline on which people are infected with the virus and then are either contagious or able to be affirmatively tested for it. In other words, if you contract the virus at noon Tuesday, it’s not the case that you will necessarily be contagious by Thursday or come back with a positive test by Saturday. It can take from two to 14 days before an infected person who is contagious or can be positively confirmed to have contracted the virus, meaning that someone could be in contact with an exposed person, test negative repeatedly for days afterward and yet still have contracted the virus — again, setting aside unusual circumstances like the one described by Maragakis.

All of which is to say Trump’s frequent tests for the virus are not a guarantee he is not contagious to others. He might have contracted the virus and be pre-symptomatic (meaning he hasn’t yet developed symptoms) or asymptomatic (won’t display any symptoms) and still be contagious. He might have tested negative because of questionable reliability on the part of the test, or he might have tested negative because he’s one of those exceptional cases where the test doesn’t come back positive.

What’s more important, really, is that screening those around Trump for the virus is itself not foolproof. While we wear masks to protect others in case we are infected, it is the case that some masks can also help protect the wearer. Doctors, for example, wear fitted masks that are designed to filter out particles as small as viruses, limiting the risk they face in treating infected individuals.

Maragakis indicated that, while not foolproof, one could wear a similar mask and more effectively protect oneself.

“Without the fit-testing, there is still the possibility that air could be breathed in around the respirator,” she said, “so it’s not 100 percent if it’s not a fit-tested respirator. But it is a higher level of protection.”

She noted such respirators have been hard to come by as the pandemic has spread. But of course, it seems fair to assume the White House could manage to secure masks offering Trump a heightened level of protection as readily as it has obtained enough testing capacity to screen those around him.

All of which is to say that, by not wearing a mask, Trump is putting others at greater risk (since he might be contagious) and increasing his own risk (by not wearing a more protective face covering that would protect him from others).

Of course, by pointedly not wearing a mask, he’s also sending a message to the American public: Do it or don’t — it’s up to you. If that message is adopted, it becomes much harder to contain the virus.

“The way I describe it is that wearing a mask is part of our social contract,” Maragakis said, “because it’s something that we can do to protect others and we are relying on others to do it to protect us. So it’s unique in that regard, where your own choice is really — it’s altruistic on behalf of someone else.”

She further recommended that, when out in public, people avoid those not wearing a mask, given the risk those individuals might pose to you. Even, presumably, if the one not wearing the mask is “the most tested man in America.”