Exercise and the Immune System

There seem to be two groups of people, those who “never get sick” and those who are chronically under the weather. Over the years, I have been a member of both camps. While I have previously suffered from overtraining syndrome and had what felt like chronic upper respiratory infections–which I wrote about in this RunFar article–I have more recently (and despite teaching high-school kids) avoided colds, flus, and other bugs. I’m sure writing that sentence will cause me to soon fall ill, though!

Why do we seem to get sick more often sometimes, and less sick in others? How does our immune system work to protect us from illness? And in what ways does exercise impact our immune system’s function? These are important questions for a community that enjoys pushing our bodies, sometimes in fairly extreme ways.

The Basics of the Immune System

I’ve mentioned this before and I think it’s worth mentioning again now: the human body is an incredibly clever system that works surprisingly well most of the time! One particularly clever element is our immune system. The immune system’s job is to protect you from outside intruders such as antigens (any toxin or foreign substance), including pathogens (bacterium, virus, or microorganism that can cause disease). Your immune system is broken into two main responses: innate immunity which is often referred to as non-specific immunity and adaptive immunity which is known as specific immunity (1).

A graphic showing how the immune system is broken down into two major response types, innate and adaptive immunity, and what those responses involve. Image: Hackney, A. C. (2013). Clinical management of immuno-suppression in athletes associated with exercise training: Sports medicine considerations. Acta Medica Iranica, 51(11), 751–756.

The innate immune system is referred to as non-specific because it mounts the same response each time no matter the type of intruder. The innate immune response includes what are known as your first and second lines of defense. The first line of defense includes not only physical barriers like your skin, but also chemical defenses like sweat, stomach acid, tears, mucus linings, and saliva. The second line of defense can be considered a chemical defense, in this case involving a variety of white blood cells. The white blood cells primarily responsible in the innate system are called neutrophils and macrophages. Both of these cell types are phagocytes, which means their job is to protect us via the process of phagocytosis (engulfing and digesting things that do not belong).

How a macrophage (the biggest of the phagocytes) identifies an intruder, engulfs it, and then uses enzymes in its lysosome to “digest” the invader. The macrophage then releases “signals” (in the form of cytokines) to sound the alarm to other cells. Image: Letstalkscience.ca/educational-resources/stem-in-context/immune-response

The adaptive immune response, called the specific response and the third line of defense, is a more complex chemical response because of how it learns to identify different antigens and is acquired over our lifetime starting from the moment we make our entrance into this world. Like the innate immune response, the adaptive response utilizes white blood cells to identify and destroy intruders, this time relying on a type of white blood cell called a lymphocyte. Lymphocytes are special in the sense that they develop memory when exposed to antigens, so that when you are exposed to them again your body is better prepared to fight them off. We develop our adaptive immunity from both natural exposures to antigens over the course of our lifetime and via other exposures like vaccines.

Image: iRunFar/Meghan Hicks

Moderate Aerobic Exercise is Good for the Immune System

Despite the field of exercise immunology being a relatively new area of scientific study with 90% of papers published after 1990, original studies date back over a century (4). Early research focused specifically on exercise-induced changes in cell counts (how many white blood cells were present before and after exercise of different intensities). From this vast body of scientific literature, we know that daily moderate exercise (up to 60 minutes of easy aerobic exercise) provides an overall “boost” to our immune system’s function, increasing our resistance to mild infections like the common cold (8). This is due to the enhanced recirculation of immunoglobulins (proteins that help recognize specific antigens), anti-inflammatory cytokines (molecules that help regulate inflammatory response), neutrophils (part of our innate immune response), and lymphocytes (part of our adaptive immune response) (4).

A graphic showing the “j curve” in which your risk of developing an upper respiratory infection decreases with moderate exercise and increases with heavy exercise. Image: Nieman, D. C., & Wentz, L. M. (2019). The compelling link between physical activity and the bodys defense system. Journal of Sport and Health Science, 8(3), 201–217. doi: 10.1016/j.jshs.2018.09.009

Can Exercise Be Bad for the Immune System?

If 60 minutes of aerobic exercise is good for our immune system, then is more better? It’s quite possible that you’ve become sick shortly after a big effort or goal race, and that’s exactly what researchers have found in widely published studies in the 1980s continuing through the present. These studies illustrate that infectious episodes (reported upper respiratory infection [URI] symptoms) increased after taking part in large endurance events. This includes a study from the 1982 Two Oceans Marathon–actually a 56-kilometer ultramarathon–where one third of the participants self-reported URI symptoms within two weeks of the race (4, 5). This and many other studies (generally conducted on major road marathon participants) helped to form the idea behind an exercise immunology theory known as the open window theory (5, 6).

The open window theory is the idea that after an intensive exercise session (either a long or hard effort) there is a period of time, generally three to 72 hours, where you have an increased susceptibility to illness. This was supported by what appeared to be a dramatic falloff of circulating lymphocytes (in particular natural killer cells, a subset of lymphocytes called T cells) post-exercise (1, 2). These values were shown to be as much as 40% below baseline cell counts (2). This was concerning because the initial studies reporting this rapid lymphocyte reduction also reported large rates of cell death (2).

That sounds bad. Well, those initial cell-death values were never substantiated–phew!–but where are the lymphocytes going if they are not dying? It turns out that lymphocytes are believed to shift to more peripheral locations in the body where they are more likely to encounter an antigen, such as in the lungs or gut (2). Think of this as white blood cell redeployment in which there is enhanced immune surveillance following strenuous exercise.

The open window theory’s premise where after intense exercise your immune system function drops below “baseline” and opens up a window of illness susceptibility. Image: Hackney, A. C. (2013). Clinical management of immuno-suppression in athletes associated with exercise training: Sports medicine considerations. Acta Medica Iranica, 51(11), 751–756.

Are we totally fine to do whatever we want, then? Not exactly. Remember, research has shown that athletes still get sick around and after major events. What we now understand now is that there is likely more than one factor at play besides just the race effort itself that causes us to get sick post-major event or effort. You also get sick when you experience repeated cycles of unusually heavy exertion, increased life stress, a decrease in sleep quantity and quality, travel (particularly air travel), and nutritional deficits (4). This sounds like a lot of the logistics involved with your goal race, right? We also understand that these confounding variables can be tricky to separate from one another, so while we know we are more likely to get sick around a long or hard effort like a race, it is good to imagine the reasons why as a perfect storm of multiple variables.

What Should Athletes Do to Decrease Illness Risk?

If we know that multiple factors influence our immune systems, what can each of us do to make sure our immune system functions properly? Address those factors!

  • Monitor your exercise workload. Adequate stress plus adequate rest equals optimal physiological improvement. But when this is thrown out of alignment and physical stress accumulates without appropriate rest, impairments in your immune system function may occur and result in an increased risk of getting sick (8). Regular exercise is good for your immune system, but a training load that is too high for too long can take its toll. Be sure to read our article on overtraining syndrome to learn more about the stress-rest balance.
  • Consider fueling strategies during exercise and avoid overall nutritional deficiency and caloric restriction. The scientific literature suggests that ingesting carbohydrates during prolonged or intense activity is associated with reduced stress hormones and reductions in inflammation. Additionally, maintaining a balanced and diverse diet that meets your needs and energy demands to match your training, including proper hydration, is important (2).
  • Practice good hygiene. Avoid close contact with individuals who are or have been sick recently, frequently wash your hands throughout the day, and avoid touching your face (nose, mouth, and eyes).
  • Practice stress management. Although short-term stress (like exercise) might have a positive effect on your immune system, chronic stress does not. Chronic stress can suppress your immune responses by decreasing the numbers and functionality of lymphocytes, and dysregulating your innate and adaptive immune responses (7).
  • Get adequate sleep. Sleep disturbances can depress your immune system, increase inflammation, and promote other poor health outcomes. Aim for seven to nine hours of sleep a night. Learn more about how sleep and your immune system interact in our in-depth sleep article.
  • Evaluate your touchpoints with others. Be considerate of how you interact with the world around you, including at running events and races. Consider fist bumps over high fives, be considerate of how you interact with race volunteers, and minimize what you touch at aid stations. (I have a habit of touching everything.) Be cognizant of not only your own health, but also the health of others. Practicing good hygiene isn’t limited to only when it’s convenient. 

Call for Comments (from Meghan)

  • Have you gone through periods where you seemed to get sick more or less often? Can you link those times to other aspects of your life?
  • Have you become ill after a major event or workout? Did it coincide with other confounding variables like travel, changes in your sleep, or more?


  1. Guimarães, T. T., Terra, R., & Dutra, P. M. L. (2017). Chronic effects of exhausting exercise and overtraining on the immune response: Th1 and Th2 profile. Motricidade13(3), 69. doi: 10.6063/motricidade.10049
  2. Peake, J. M., Neubauer, O., Walsh, N. P., & Simpson, R. J. (2017). Recovery of the immune system after exercise. Journal of Applied Physiology122(5), 1077–1087. doi: 10.1152/japplphysiol.00622.2016
  3. Hackney, A. C. (2013). Clinical management of immuno-suppression in athletes associated with exercise training: Sports medicine considerations. Acta Medica Iranica51(11), 751–756.
  4. Nieman, D. C., & Wentz, L. M. (2019). The compelling link between physical activity and the bodys defense system. Journal of Sport and Health Science8(3), 201–217. doi: 10.1016/j.jshs.2018.09.009
  5. Campbell, J. P., & Turner, J. E. (2018). There is limited existing evidence to support the common assumption that strenuous endurance exercise bouts impair immune competency. Expert Review of Clinical Immunology15(2), 105–109. doi: 10.1080/1744666x.2019.1548933
  6. Campbell, J. P., & Turner, J. E. (2018). Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan. Frontiers in Immunology9. doi: 10.3389/fimmu.2018.00648
  7. Dhabhar, F. S. (2014). Effects of stress on immune function: the good, the bad, and the beautiful. Immunologic Research58(2-3), 193–210. doi: 10.1007/s12026-014-8517-0
  8. Inkabi, S. E., Pushpamithran, G., Richter, P., & Attakora, K. (2017). Exercise Immunology: Involved Components and Varieties in Different Types of Physical Exercise. Scientect Journal of Life Sciences.

Image: iRunFar/Meghan Hicks

There are 10 comments

  1. Martin

    Thanks for the great discussion on how immune systems work!

    For years, I always came down with a couple of serious (day or two off from work) colds per year. This stopped about 6 months before retirement 3+ years ago, after which I no longer had any illness at all. Until I participated in the Across the Years 10 Day event held a few months ago in Dec-Jan, and caught a cold about half way through. So, yes, stress and travel related. Change in sleep patterns? Hahaha!

    A multi-day event is perfect for seeing what your immune system is worth. I ran ATY 6 Day the previous year with no ‘ill’ effects. Other than feeling trashed for a month!

  2. Sam

    When I was training for cycling (crits & road races) a constant challenge for me was coming down with flu/cold type illnesses after training in high intensity zones. This would have a cascade-effect of disrupting my training cycle.

    For me, I think a lot of it was too many late nights with alcoholic drinks. I didn’t necessarily party hearty, but it would be hard to say no to going out and socializing with friends, usually at a bar or party or whatever.

    It’s hard when the people in your social circle are not endurance athletes and don’t really understand that you’re really pretty wrecked after hard training. And of course, inevitably, there is never anything going on socially when it’s a recovery week, only during hard weeks!

  3. Ryan Montalvo

    This is a really great article, I really appreciate the research that went into this. I hate using this phrase but “in these times” we need to continue to exercise and take steps to make sure we stay healthy. The information about the time frame of when you are more susceptible to getting sick after an intense work out is great to know. I did not think the time frame would be so long. Your recommendation are also spot on, especially sleep. Being well rested in the key to staying healthy. Thank you.

    1. Corrine Malcolm

      Hi Ryan,
      We intentionally tried to steer away from making it explicitly about “Covidian Times” and more general on exercise and the immune system so it would stand the test of time (at least for now!). What I found most interesting is that the window in which you are more susceptible is actually much shorter than what we thought during the 80’s-90’s and even into the early 2000s, which is that 3-72hour window. With proper sleep, adequate nutrition and hydration, etc the window is likely negligible. So just like you mentioned, keep focusing on controlling the ‘controllables’, taking care of yourself in and around training and racing.

  4. Jane

    I’m curious how you feel about long endurance activities in the age of coronavirus. I admire your work but feel as though it’s hypocritical to see athletes partaking in these enormous activities in the name of “running for covid” when it seems counter productive to all the public health guidelines. Ie. Crewing for friends you’re not practicing social distancing with, running in areas not local to you (or your backyard), amassing large groups of people to cheer on these Herculean efforts…If we cared about helping frontline workers we’d volunteer directly, not compromise our immune systems running for hours on end, no? How do you remedy this? Your science is spot on, it just feels a bit hypocritical.

    1. Corrine Malcolm

      Hi Jane,
      Thank you for your thoughtful commentary, you are spot on. We however, intentionally did not talk about “Covidian Times” during this article to make it more about general exercise/racing and the immune system, so that even in 12 months or 3 years you could hopefully read this article and find the information pertinent. It’s hard to make recommendations broadly, specifically about exercise/racing during Covid-19, because restrictions and the current state of things are so different depending on where you live (and also the individuals involved). I’ve personally volunteered with PPE and blood drives here in SF, my running partners are who I live with and who I work with (I’m lucky they’re runners), and I’ve opted out of challenges that might be risky or impede my health (and therefore burden the healthcare system) like choosing to not Everest Mt Tam right now despite really wanting to. Can you safely run during Covidian times, certainly (with an * being it’s dependent on where you live). Can you safely run long distances during this time, again certainly, it turns out our immune systems are far more robust and the “open window” is very small… but again this will be dependent to the individual and where they live. We as individuals and as a community are going to have to make hard personal choices over the coming months as races and events get added back to calendars. ie traveling out of state or internationally for a race? Heck, I’m suppose to get married in September…. We’re not always going to get it right, but I do think people in the trail and ultra running community, overwhelmingly are trying their best with the knowledge we currently have.

  5. Andrew

    Hi Corrine great article. What are your views on long term impacts of ultra running? I did Ronda Del Cims last July is 60 hours or so. Probably 1 hour of really poor sleep over the two nights.

    I still feel tired now when I run and wonder if I have done longer term impact on my immune system. interestingly I don’t really get unwell just struggle to feel fit when running and feel beat up afterwards.

    My longer term goal is Tor Des Geants so be interesting to understand if I’m potentially doing longer term damage from such races

  6. Joshua Michael Moreau

    Hi Andrew,
    I thought I’d comment as I am both a research immunologist and have observed first hand the impacts of sleep deprivation during 200 mile races. It’s a really interesting question!

    Absolutely, sleep deprivation and generally poor sleep patterns are negative for immune function. However, it may not be immune repression but rather immune dysregulation leading to unwelcome inflammation. Recently, a really interesting preclinical (animal model) report came out showing that chronic sleep fragmentation caused profound changes that exacerbated atherosclerosis. The key thing to note here though is that was a very chronic stimulus. If you only get 1 hour of sleep over a couple of days but normally sleep well then I think you’d be okay. In fact the immune response to loss of sleep probably evolved to be protective during just such an event. Lingering fatigue is possibly a sign that the body still needs full recovery (both from the 60 hours of running and the poor sleep). Which circles back to the chronic aspect of this. Generally, in modern society people do not get enough restful sleep or otherwise prioritize recovery. I think when doing super long events careful recovery is even more important.

    When thinking about something like Tor Des Geants I wonder if it would help to specifically train for sleep deprivation? I’m not aware of any particularly clear research on this but it seems possible that it would be trainable. Optimizing a sleep strategy for such a long race likely would improve run performance but also long term recovery.

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