Marathons have quickly become a popular pursuit. Hundreds of thousands of people submit ballots each year to run in some of the most prestigious races. In 2024, a record number of people crossed the finish line at some of the world’s biggest marathons. If you haven’t run a marathon yourself, chances are you know someone who has.

While we know that running has a range of health benefits, including reducing your risk of illness, research also shows that runners are more likely to catch a cold after completing a marathon.

Scientists used to think this was caused by reduced immune function following prolonged exercise, alongside increased exposure to infection due to the large crowds that marathons attract. But our latest research shows that a person’s anxiety, stress and mood may also play a role in whether they’re more vulnerable to getting ill or not after a long run or marathon.

In our first study, we asked 406 runners to complete questionnaires about their anxiety and mood in the month and days before running a marathon. We also asked them to provide saliva samples before and after the race.

The mouth is one of the main routes of access for the majority of viruses and bacteria that cause respiratory infections. The saliva samples allowed us to detect whether there were signs that the defence systems that normally stop these pathogens from entering the body (known as mucosal immunity) were suppressed.

We then tracked their common cold symptoms during the two weeks after their marathon.

Runners who were more anxious generally and those who experienced greater mood disturbance before the race (such as anger, frustration or tension) were more likely to experience a respiratory infection during the two weeks after the marathon. They also had a greater reduction in mucosal immunity.

In the second study, we asked 45 adults to run on a treadmill for an hour inside our laboratory. We asked them to complete questionnaires about their anxiety, stress and mood before they completed the run. We also measured their mucosal immunity in saliva before and after the run.

Men with higher anxiety levels, stress levels and greater mood disturbance were more likely to have a reduction in mucosal immunity after the run compared to men who had lower anxiety and stress levels before the run.

When examining the influence of stress, anxiety and mood on the immune response to one hour of running in women, findings were not as clear cut. There are many potential reasons for this – with factors such as hormone fluctuations, contraceptive use and differences in immune response depending on menstrual cycle phase all potentially having an influence. It will be important for future studies to examine this.

Together, the findings from our studies indicate that people who are more stressed or anxious before a run might be at greater risk of getting sick or catching a cold. This effect seems to apply not only to marathon-length runs, but to moderate-length runs of around an hour, too.

Immune function

One possible reason for this link between stress and immunity is due to the way stress changes how the immune system functions.

Both psychological and physical stressors affect the body through similar mechanisms – specifically through the hypothalamic-pituitary axis and sympathetic-medullary axis. These pathways link the nervous system to the brain and play a role in the stress response. High psychological stress or prolonged exercise can suppress these axes and reduce immune function.

So when runners experience both psychological and physical stress, the impacts on the immune system might be more significant than if they were just experiencing psychological or physical stress alone.

For example, both mucosal immunity and the immune system’s ability to respond to new foreign pathogens are reduced following prolonged running in people with higher anxiety and stress levels. This shows just how significant the effects of both psychological stress and physical stress are when it comes to immune function.

But before you cancel that 10k or withdraw your marathon ballot, it’s important to remember that being physically active still reduces your risk of a respiratory infection compared to not exercising at all.

Physical activity also reduces risk of many other severe health conditions including cardiovascular disease, cancer and diabetes. Running can also be beneficial for reducing psychological stress and anxiety.

Instead, it’s important to prepare yourself well before your run by dealing with your stress and anxiety beforehand. Finding ways to reduce stress before a run should be treated the same way as you would ensure that you’re hydrated and fuelled well.

Some things you can do to reduce stress include relaxation exercises (such as breathwork, mindfulness or yoga) and getting a good night’s sleep.

It can also help to monitor stress, anxiety and mood so that you can identify when your stress or anxiety starts to increase – or when your mood begins to worsen. This will help you identify triggers and take proactive steps to reduce the impacts before things progress and become worse.

If you’re taking part in a race or marathon, some easy ways to reduce stress include:

• Planning your travel route in advance and allowing extra time to avoid rushing and stress
• If you can, try to arrive the night before the race to scope out your route
• Plan some “what if” or “if/then” strategies to be prepared to deal with anything unexpected on race day.

Of course, some of the best ways to avoid getting sick involve sticking to the tried and tested techniques – such as washing your hands properly (and avoid touching your eyes, nose and mouth), aim to get at least seven hours of sleep each night, eat a well-balanced diet, plan a recovery week into your training every second or third week and never train if you have an injury.