Why do I exercise? Until two weeks ago, I’d have told you that I work out to keep my back in shape, my brain functioning, and to avoid developing a resemblance to an overstuffed couch.

After the events of the last few weeks, in the midst of this COVID-19 crisis, I have a different answer. Read this, and you might, too.

Right now, I’m exercising to give my immune system the boost it needs to work properly while it’s under attack. Because frankly? This is war.

As you’ve probably noticed, anyone can go down with this virus — young, old, and especially middle-aged. Ultimately, most of us will be infected. But there’s a difference between who stays down, and who gets back up, and how fast that happens. While we’re all social distancing and washing our hands and spraying bleach on our groceries, it’s easy to forget that a strong immune system can make the difference between a mild case of COVID-19 and one that wreaks systemic havoc.

Cathryn Jakobson Ramin and her book Crooked

You’ve heard the talk about the increased vulnerability to COVID-19 of people with comorbidities such as Type-2 diabetes, hypertension and heart disease, all of which have soared in prevalence in the U.S. over the past two decades. In many cases, these disorders are the offspring of insufficient aerobic exercise, unhealthy diet, psychological stress and inadequate sleep. In other words, they are avoidable. With about 40 percent of the population currently obese — in itself a risk factor for severe infection — and about the same percentage acknowledging that they are largely sedentary, it’s no wonder that Americans are succumbing to COVID-19 in numbers that drastically outpace the per capita mortality rate in other affected countries.

What we know is that being physically fit and active makes us better able to resist infection. The risk of getting a cold or a flu, or having a particularly severe form of the infection drops by almost half if you engage in 30 to 60 minutes of moderate exercise at least five days a week. A couple of caveats: If you’re just getting started, don’t exhaust yourself with an intense and long workout, because that kind of extreme stress could temporarily depress your immune system. And if you’re already sick, go very easy until your body recovers.

Related: CARDIOVASCULAR EXERCISE: When and How to (Re)Start

Why does exercise work?

Our immune system has two sets of defenses against viruses and other pathogens. There’s a first-line army of cells, called leukocytes, that attack invading microbes within minutes to hours. Consider these cells the foot soldiers in the war against any kind of infection.

There’s a second-line force of T cells and precisely targeted antibodies (think: heavy duty artillery). The T cells are loaded with virus-fighting chemicals that follow the leukocytes, arriving at the front lines up to a couple of days later. Ideally, the leukocytes already have the situation well in hand, and the antibodies and T cells only stick around long enough to vanquish stray combatants.

The problem is that as we age, our T cells are not the impressive fighting machines they were when we were young. T cells that have been programmed, typically via vaccination, to fight diseases like smallpox and cholera, remember what they were taught, and offer protection for decades. But “naive T cells” — those that were never recruited for a specific fight — gradually deteriorate and fall into “immunosenescence,” explaining the increased susceptibility of older people to COVID-19.

Here’s the kicker: New studies suggest that exercise helps the body rid itself of T cells that have lost track of their mission, and replace them with T cells that are ready to go to work, constantly re-stocking the immune system’s arsenal of ammunition. That’s the most valuable weapon we have in this fight — and the only way to stay battle-ready is to keep exercising. If you’ve taken a break — and who hasn’t been tempted in the last month — I suggest that get out your gym mat, and work out with Jeremy James at BACKFOREVER. Afterwards, put on some music and dance your head off.

Your back will thank you. And so will your immune system.

To help us navigate these difficult times, we are pleased to welcome Cathryn Jakobson Ramin, author of the widely acclaimed CROOKED: Outwitting the Back Pain Industry and Getting on the Road to Recovery, as a guest blogger. In this new series, she’ll provide insight and valuable tips for staying active and pain-free at home.