Last week my sister-in-law who lives in a tiny village outside of Trieste was taking her daily walk in the country when she was stopped by a heavily armed soldier. He threatened her with arrest unless she immediately returned to her house and stayed inside.

Now California Governor Gavin Newsom has issued a “shelter in place” order, under which “all non-essential travel” on foot, bicycle, scooter, automobile, or public transit “is prohibited.” Although the order allows outdoor exercises like walking, jogging, and bike riding in the open air—at a safe distance from others—that fine print can too easily be overlooked. Which would be a shame, not just on public health grounds.

For the tens of millions, young and old, whose schools or workplaces are closed, daily exercise is essential to maintaining strong immune systems. “Stay at home” should not mean “stay inside.” While exercises that bring people into close contact with one another—such as team sports and weight-training at the Y—are a bad idea, we should encourage people to spend their downtime taking solitary walks and bicycle trips, or for that matter hikes in the country that avoid state campsites and national parks. Public space, used responsibly, is an important medicine.

At a recent press conference, President Trump urged people to simply watch more TV. He, of course, prospers from our passivity and powerlessness. And because he doesn’t read books, he didn’t bother to mention opening a good book as an alternative to watching Fox News or listening to Sean Hannity rant nonsense on the radio.

Generations X and Z need no advice from me, since they are already on social media 24/7, exchanging good information and bad, but also maintaining healthy social interactions. It is an entirely different story for those who are older, sicker, and alone. Their social isolation can become life-threatening.

Indeed, the combined effects of fear, confinement, income loss, and the potential destruction of family savings augur a mental health crisis on an even larger scale than the pandemic itself. This isn’t simply collateral damage, but rather an integral and extremely dangerous part of the health threat that has so far been neglected. Solidarity is an essential vaccine.