It seems that 2020 was the year we all became armchair immunologists, as we got a daily crash-course in how the body works to defend itself against a variety of threats. It hasn’t been pretty, but it has made many people more aware of how important their immune system is and how we can help support it.

Viruses, such as the novel coronavirus that has completely shaken up our world, have been front and center lately. But there’s a vast and complex constellation of pathogens in the environment that can enter your body and cause illness. Your immune system is there to help prevent bacterial, fungal, and other potentially problematic agents from running rampant and making you very sick.

One of the best ways you can support your hard-working immune system is by exercising. Any kind of exercise can boost the performance of your immune system—to a point. If you work to the point of exhaustion, that can actually diminish your immune system’s performance. But there’s a lot of runway between here and there, and by and large exercise—especially aerobic exercise—can support a healthy immune system.

Taken one step further, there’s a body of evidence that suggests that swimming—particularly in cool or cold water—has a special advantage in supporting immune health. In fact, swimming may be one of the top options available if you’re looking to support your immune system via exercise. There’s just something about being submerged in an environment that’s colder than your body temperature that stimulates your immune system to step up its game.

There are a number of reasons for this, and many of these apply both to open water and pool swimming.

Contrary to the old wives’ tale that going outside in the winter with wet hair will cause you to come down with a cold or the flu, swimming in water cooler than your body temperature can actually increase your ability to resist an infection.

According to an article on Harvard University’s medical school website, “Exposure to moderate cold temperatures doesn't increase your susceptibility to infection.” It’s just that simple—pathogens cause disease, not temperature. It’s not the cold that makes fall and winter cold and flu season, it’s how people congregate indoors in the colder months that spreads these seasonal diseases.

More to the point, getting wet in winter could hold an important key to health and longevity. One reason for that is because cold water swimming has been shown to boost the number of white blood cells—part of the immune system—that are circulating in your blood.

White blood cells are a component of your immune system that helps your body fight off infection. They’re manufactured in your bone marrow and circulate throughout your body via the blood and lymph tissue and fluid. You need them to fight infection, so you want a healthy level of these important cells in your body.

Your body produces more white blood cells when faced with a stressor, and swimming in cold water stresses your body in such a way as to stimulate their release. A 2011 study found that swimming in cold water produced significant increases in white blood cell counts as well as other components of blood, including platelets and red blood cells.

Other studies have found that swimming regularly in cold water makes these effects linger longer. That means that your body will have more soldiers at the ready when an invading viral army shows up.

As with virtually any type of exercise, swimming releases endorphins, the hormones responsible for the feelings of “runner’s high” that swimmers also experience after an intense workout. Cold water can release these sensations faster and with less intense swimming than regular water, but make no mistake, pool swimming, even in 82-degree water, can induce that same euphoria. The more intense the workout—whether that’s trying to hold the intervals you did in college or shivering through a short jaunt in the harbor—the more it can boost the performance of your immune system.

Exercise and swimming helps keep your blood moving, which is great for a number of aspects of overall health. When your circulation improves, that boosts your body’s ability to move oxygen and nutrients to cells that need them and to remove waste those cells produce to keep things running at top efficiency.

When it comes to your immune system, that improved circulation also delivers nutrients to your bone marrow, which is your immune system headquarters. Efficient circulation also helps remove waste, which can also reduce inflammation and help your immune system do its important work.

Stress is a double-edged sword. In limited amounts, it can be a great way to make yourself sharper, stronger, and better. Indeed, every training modality is built on the model of adding a controlled amount of stress on your body in an incremental way to stimulate growth and improvement.

But when bad stress—such as what many of us feel at work or when in conflict with others—builds and become chronic, that can become an unhealthful situation that can have long-term implications.

Chronic stress can be a major source of systemic inflammation throughout your body, and as that inflammation grows and accumulates, that can overwhelm your body’s natural defense mechanisms and set you up for chronic disease such as diabetes and cancer.

Exercise, especially high-intensity cardiovascular exercise, provides an outlet for some of this stress, which can in turn lower inflammation levels throughout your body. That may lead to lowered risk of chronic disease. If your immune system isn’t compromised with all that inflammation, that means it will be more readily available to attack the next invading pathogens and to support your general awesomeness.

As much as you want to boost your intake of vitamin C and echinacea in the winter, maybe adding a few swims a week would pay even bigger dividends.

Elaine K. Howley is an award-winning freelance writer and editor specializing in sports, health, and history topics. Her work has appeared in numerous print and online publications including  AARP.org,  Atlas Obscura,  espnW, and  U.S. News & World Report. A lifelong swimmer who specializes in cold water marathon swimming, she has contributed to  SWIMMER magazine since 2009 and  USMS.org since 2012. Contact her via her website:  elainekhowley.com

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