Can exercise make you sick? Where the exercise and immunity debate stands

Can exercise make you sick? Many people have a story about catching a cold right after a hard race or a heavy week of training. Whether the exercise itself is to blame is an open argument in sports science, and the two camps have spent years disputing it in print. Their most recent joint paper is a debate article, and it ends with a list of what both sides accept and what is still undecided.

This article goes through four sources on exercise and immunity. It is informational material, not medical advice, and it says nothing about how to train. If symptoms are severe, getting worse, or not going away, that is a question for a doctor.

Can exercise make you sick? The older view

The idea that heavy exertion weakens defences against infection has a long history. A 2019 review of the field by Nieman and Wentz sorts its research into periods. In the 1980s, they write, seminal papers appeared with evidence that heavy exertion was associated with transient immune dysfunction, raised inflammatory markers and an increased risk of upper respiratory tract infections (Nieman & Wentz, 2019).

One of the best-known studies of the time came from a marathon. In 1990 Nieman and colleagues surveyed applicants to the Los Angeles Marathon. They randomly picked 4,926 of 12,200 applicants and sent them a four-page questionnaire. 2,311 replied. Everything in it was self-reported.

Two numbers came out of it:

• Runners who trained 97 km a week or more, against those under 32 km a week, had 2.0 times the odds of reporting an infectious episode in the two months before the race (95% CI 1.2 to 3.4).
• Of 1,828 participants with no infection before the race, 236 (12.9%) reported one in the week after it. Among 134 similarly experienced runners who did not run the marathon, 3 (2.2%) did. The odds ratio was 5.9 (95% CI 1.9 to 18.8).

The authors adjusted for age, marital status, sickness among others in the household, perceived stress from the training regimen and the effect of sickness on regular training. Their conclusion was cautious: the data suggest runners may have higher odds of an infectious episode during heavy training or after a marathon (Nieman et al., 1990).

The open window theory, and the review that challenges it

The "open window" idea says that after a hard session there is a period when the immune system is suppressed and infections can get in. A 2018 review by Campbell and Turner goes through the evidence behind it and finds three weak points (Campbell & Turner, 2018):

1. Little reliable evidence shows that vigorous exercise raises the risk of opportunistic infections.
2. Changes in salivary IgA after exercise, often cited as a sign of weakened mucosal defence, do not signal a period of immune suppression.
3. The sharp fall in lymphocyte numbers one to two hours after exercise reflects a temporary redistribution of immune cells to peripheral tissues. The authors read it as heightened surveillance, not suppression.

They add that frequent exercise appears to improve immune competence, and that human vaccination studies show stronger responses to bacterial and viral antigens after bouts of exercise. Their summary is blunt: labelling any form of acute exercise immunosuppressive is a misconception.

That is one side of the argument, and the review was written to rebalance opinion, as its authors say. It does not claim that nobody gets sick after exercise.

What both camps accepted in the debate paper

In 2020 eight researchers published a debate article titled with the question itself: can exercise affect immune function to increase susceptibility to infection? Two of the authors, Campbell and Nieman, also appear above: Campbell co-wrote the 2018 review, Nieman the 2019 history and the 1990 survey. The paper was built to collect opposing arguments, and its abstract reports where they landed (Simpson et al., 2020).

Where they agree:

• Regular bouts of short, up to 45 minutes, moderate-intensity exercise are beneficial for immune defence. This holds particularly for older adults and people with chronic diseases.
• Infection burden is reported to be high among high-performance athletes. After injury, it is the second most common reason for lost training days before major events.
• Susceptibility to infection has a multifactorial basis.

Where they do not:

• Whether athletes are more susceptible to infection, mainly of the upper respiratory tract, than the general population.
• Whether exercise on its own can change immunity enough to raise infection risk, independently of the many other things these groups go through. The paper lists anxiety, sleep disruption, travel, exposure to others, nutritional deficits and environmental extremes.
• How useful certain biomarkers are, and how to interpret in vitro and in vivo data in people doing arduous exercise.
• The quality of the evidence each side uses to support its claims.

The abstract states plainly that whether exercise per se is a causative factor in increased infection risk in athletes remains to be resolved.

Why the marathon data do not settle it

Put the survey next to the debate paper and the gaps in the survey are easy to see. Reports of infection came from a questionnaire, not from a swab or a doctor's record. Respondents differed from the other race finishers: they were 2.0 years older and 7.6 minutes faster. The comparison group, runners who did not enter the marathon, was small, 134 people, which is why the confidence interval around 5.9 is so wide.

More basically, a marathon week can bundle together the kind of factors the debate paper lists: anxiety, disrupted sleep, travel, exposure to other people. The survey cannot say which of these, if any, mattered, or whether it was the running. Its authors wrote only that the data "suggest" higher odds.

The same applies in reverse. A review that shows lymphocytes moving to tissue after exercise does not show that nobody's defences ever dip. The evidence on each side carries its own limits.

What nobody knows yet

• Whether exercise as such causes infections in athletes, or whether other factors around training do. The debate paper leaves this open.
• Whether athletes get more upper respiratory infections than the general population. The two camps contested it.
• Whether the post-marathon rise in self-reported infection would survive laboratory confirmation. The 1990 survey did not confirm any case.
• Whether any of this applies to someone who does not train at the level of a marathon or high-performance sport. The sources above mostly discuss runners, athletes and people doing arduous exercise. The consensus on moderate exercise comes from the debate paper's agreed points, and the abstract does not give figures for it.

What you can look at in your own data

The app reads workouts from Apple Health and compares their duration and intensity with your sleep and wellbeing, and the number of workouts with your energy. Resting heart rate and heart rate variability from Apple Health are there too. It records nine fixed symptoms: headache, fatigue, drowsiness, dizziness, anxiety, irritability, nausea, joint pain and insomnia. Cough and temperature are not among them. The app does not recognise infections and does not diagnose them, so it cannot tell you whether a cold followed a workout. It can show how a hard week looks next to your sleep and energy entries.

For what else is linked to catching colds, see fatigue and getting sick, immunity: what research links to it, and what is marketing and sleep and catching a cold.