I have always struggled with the belief that aerobic exercise can lead to a decrease in the ability of the immune system to function. I just didn’t see how something like exercise, which benefits our bodies in almost every system, could somehow be damaging to our integral immune system.

There were many studies throughout the 80s, and into the 90s, relying on athletes to self-report instances of illness (an inaccurate system of measurement), that led us to believe that this was the case. As endurance races would be going on, the bloodstreams of the athletes would be chock full of immune cells. This continued until a few hours after the strenuous endurance exercise was completed and suddenly the immune cells would vanish to the point of their levels being lower than before the race even started. Scientists believed that this meant the strenuous activity had killed the immune cells, leaving the athletes’ bodies vulnerable to infection.


Fortunately, as of April this year, the truth of this issue has finally been brought to light. Scientists at the University of Bath in the United Kingdom performed a study on endurance athletes, testing their saliva for markers that would indicate an illness. They found that upper respiratory illnesses that are generally reported by athletes are often misreported. After a marathon the athletes were tested in the following weeks and were found to have the same instances of illness statistically as the rest of the city the marathon was hosted by. Further tests were done on mice, where scientists dyed the immune cells in the mouse bodies and were able to follow them as they moved throughout.

The findings were that during exercise the immune cells move into the bloodstream and are transported around the body. Once exercise is finished, they are brought into the gut and lungs to help with immune function in areas of the body that are vulnerable after strenuous physical activity. Some of the immune cells are also transported to the bone marrow to initiate production of additional immune cells from the stem cells within. This shows us that exercise is good for the immune system in mice, so there should be benefits in the human immune system as well.

              Now you may be asking, “why should we care what happens inside the bodies of rats and mice?”. That’s a great question, and one that I used to ask myself all the time when doing research. The majority of studies I read involving humans always seemed to either include or refer to rodent studies.  Finally, the curiosity built to a breaking point and I went back to the books.

It turns out that the genetic and biological make up of rodents are very similar to those of humans. In fact, mice and humans have 4/5 of their genes in common with each other [5]. Because of this, we can infer results and effects on these creatures and apply them to humans. It’s not incredibly kind sounding, but rodents are also very inexpensive, can be bred to have almost uniform genetic make up to remove variance, and are small, which makes it easy to house and handle them. In fact, 95% of all animals used in lab research are mice and rats [5]. There is actually a monument to laboratory mice in Novosibirsk, Siberia, Russia. We do certainly owe them a lot for the vast knowledge they’ve helped us to obtain.

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Now, what about once you’re already afflicted with an illness?

It turns out that there is plenty of information on what to do in this case. First of all, I would like to state that exercise levels should always be decreased when you’re sick. You exercise to how well you’re feeling, don’t push yourself to what you normally do.

Mild to moderate exercise is perfectly fine when the symptoms of your illness occur above the neck, but intensity and duration should be (again) decreased. Strenuous exercise is NOT okay if your symptoms are below the neck (chest congestion, hacking cough, upset stomach). It should also be decreased if there is a fever, or widespread muscle cramps or weakness. Use your body as a guide, as your condition improves you may begin to resume your regular activity levels. This being said, exercise is good for your immune system, and if you’re feeling up to it, getting up to walk and move about is not going to seriously harm you, and should actually help to circulate your immune cells, as long as you don’t push yourself [2].

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How much is too much under normal circumstances?

When it comes to aerobic exercise, as long as you’re not running a total weekly distance greater than 42 km, your incidence of upper respiratory illness should be improved [3]. So, if you run more than a marathon a week you need to decrease your training levels, which is not something most people have to worry about.

Although it seems that the most immune function benefit stems from moderate aerobic exercise, a combination of activities is the best plan to get an assortment of benefits from your body. Include strength, aerobic, and anaerobic activities into your weekly regimen. Use a combination of moderate, leisurely, and vigorous exercise. As long as you’re getting out there, you’re bettering yourself in more ways than just one. Some of these are, a decrease in upper respiratory illness, a decrease in heart disease, increased bone health and strength, improved cognitive function, and slowed release of stress hormones. It may still be unknown to scientists how immune function is improved through exercise, but what is not up for debate is the fact that it is improved.



1.       Gretchen Reynolds. How Strenuous Exercise Affects Our Immune System. (April 2018)


2.       Edward R. Laskowsk, MD. Is It Okay to Exercise if I Have a Cold? (February 2017)


3.       Thomas G. Weidner, PhD, ATC; Thomas L Sevier, MD. (1996) Sport, Exercise, and the Common Cold. National Center for Biotechnology Information (Volume 31)


4.       Linda J. Vorvick, MD. Exercise and Immunity (November 2018)


5.       Remy Melina. Why do Medical Researchers Use Mice. (November 2010)