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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

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).ow 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.

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).

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.

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.

Most Olympic marathoners spend their youth focused on running. They join track clubs, compete in national championships, and pursue the sport from an early age.

Julia Mayer’s journey was very different.

Today, Mayer is Austria’s marathon record holder, a multiple national record holder on the roads, and an Olympian. Yet for much of her athletic life, running was not her primary sport.

For 16 years, she played soccer.

Then she discovered something that would change her life.

“I noticed that I was really fast in the fun runs and that it was really, really fun,” Mayer said when reflecting on her transition from soccer to distance running.

What began as curiosity quickly became a passion. She eventually made the bold decision to leave soccer behind and focus entirely on running. It was a move that surprised many people around her, but Mayer believed she had found her true athletic calling.

The decision proved to be the right one.

Within a few years, Mayer developed into one of Europe’s top marathon runners. Her steady improvement carried her from local races to the international stage, where she began rewriting Austria’s record books.

She now holds Austrian records in the marathon, half marathon, and road 10K. Her marathon best of 2:26:08 established her as the fastest female marathoner in Austrian history. Her performances in the half marathon and 10K have further cemented her place among the country’s all-time great distance runners.

Her rise culminated with qualification for the 2024 Olympic Games in Paris.

Competing in the Olympic marathon represented the realization of a dream. On one of the most challenging marathon courses ever used for the Olympics, Mayer ran courageously against the strongest field in the world and finished 55th in her Olympic debut.

Behind the scenes, success has come through extraordinary dedication. During marathon preparation, Mayer trains twice a day and covers approximately 200 kilometers, or 124 miles, each week. The workload demands discipline, patience, and a deep commitment to continuous improvement. 

What makes her story especially inspiring is not simply the records or the Olympic appearance.

It is the fact that she found her greatest talent later than many elite runners.

In a sport where athletes are often identified at a young age, Mayer’s journey serves as a reminder that potential does not always reveal itself early. Sometimes it takes years of experience, a willingness to try something new, and the courage to follow a different path. 

The former soccer player who once chased a ball across a field is now chasing history on the roads of Europe.

And according to those closest to her, her best performances may still be ahead.

For runners of every age and ability, Julia Mayer’s story delivers a powerful lesson: it is never too late to discover what you are capable of.

From soccer player to Olympian, her journey proves that remarkable achievements can begin when least expected.

Megan Keith produced the performance of her career in Oslo on Thursday night, shattering one of Scotland’s longest-standing distance running records and cementing her place among Britain’s greatest female 3000m runners.

The 24-year-old clocked a sensational 8:28.35 over 3000m, breaking the Scottish outdoor record that had stood for nearly four decades. In doing so, Keith eclipsed the previous mark of 8:29.02, set by Scottish legend Yvonne Murray back in 1988, ending a record reign that had lasted 38 years.

Keith’s breakthrough run was more than just a national record. The performance also propelled her to third on the UK outdoor all-time list, placing her behind only two of Britain’s most celebrated distance runners — Paula Radcliffe, who leads the rankings with 8:22.20, and Laura Weightman, whose 8:26.07 remains the second-fastest outdoor mark by a British woman.

The significance of Keith’s achievement is amplified by the calibre of athletes she now joins in the record books. For decades, Murray’s mark stood as one of Scottish athletics’ most untouchable records, surviving generations of elite competitors. Keith has now succeeded where many outstanding runners have fallen short, announcing herself as one of the leading distance talents in British athletics.

Her time also compares favourably with the best performances produced indoors. Olympic medallist Laura Muir ran 8:26.41 indoors in Karlsruhe in 2017, underlining just how exceptional Keith’s outdoor effort in Oslo truly was.

The run continues a remarkable rise for the Scottish star, whose progression over recent seasons has transformed her from a promising prospect into a genuine force on the international stage. Running with confidence and composure against elite competition, Keith demonstrated both the speed and endurance required to challenge the very best in Europe and beyond.

With the World Championship season gathering momentum, Keith’s record-breaking display sends a powerful message. Not only has she etched her name into Scottish athletics history, but she has also established herself as a serious contender in one of the sport’s most competitive events.

In Oslo, Megan Keith did far more than break a record. She ended a 38-year wait, climbed into the upper echelon of British distance running, and delivered a performance that may prove to be a defining moment in her career.

British middle-distance talent Phoebe Gill took another significant step forward in her return to top form, producing a determined victory over 800 metres at the Meeting de Marseille in France on Wednesday.

Competing in challenging, wind-affected conditions, the 19-year-old demonstrated both resilience and composure as she held off a late charge from Switzerland's Veronica Vancardo to secure the win in 2:00.81. Vancardo finished just three hundredths of a second behind in 2:00.84, underlining the fiercely contested nature of the race.

While the margin of victory was narrow, the result represented another encouraging milestone for Gill as she continues to rebuild momentum following her injury setback. The young Briton showed impressive race awareness and strength in the closing stages, maintaining her advantage despite the difficult conditions that made fast running a challenge throughout the evening.

The Marseille triumph adds to a growing body of evidence that Gill is steadily progressing toward her best form. Earlier in her comeback campaign, she clocked 2:01.50 for 800m in Bydgoszcz before demonstrating her versatility with a strong 4:05.53 performance over 1500 metres at the BMC Grand Prix meeting in Trafford.

Those performances have highlighted not only her improving fitness but also her ability to compete across multiple distances as she carefully builds her season. The Marseille victory now provides further confirmation that the European junior star is moving in the right direction.

Gill emerged as one of Britain's most exciting middle-distance prospects through a series of breakthrough performances as a teenager, earning widespread recognition for her fearless racing style and remarkable maturity. Injury temporarily interrupted that upward trajectory, but her recent results suggest she is steadily rediscovering the form that made her one of the sport's brightest young talents.

With each race, the signs of progress become increasingly evident. Winning in difficult conditions and under pressure from a quality field is often a stronger indicator than a fast time alone, and Gill's latest success demonstrated exactly those qualities.

As the summer season gathers pace, the Marseille victory offers another confidence boost for the British teenager, whose return continues to gain momentum. If her recent progression is any indication, Gill could soon find herself back among the leading names on the European middle-distance circuit. 

The NCAA Track and Field Championships delivered a moment few could have predicted as Ja’Kobe Tharp produced one of the most astonishing performances in the history of sprint hurdling, rewriting the record books in spectacular fashion.

Competing in the opening round of the men’s 110-meter hurdles, the Auburn star stunned the athletics world by clocking an incredible 12.75 seconds, becoming the first athlete ever to break the 12.80-second barrier. In doing so, he eclipsed the long-standing world record of 12.80 set by Aries Merritt in 2012, a mark that had stood as one of the sport’s most revered achievements for more than a decade.

What makes Tharp’s breakthrough even more remarkable is the scale of his improvement. Entering the championships, the defending NCAA and U.S. champion had recorded a season-best of 13.05 seconds. Yet under the brightest spotlight, he unleashed a performance that exceeded every expectation, slicing an extraordinary 0.26 seconds from his personal best in a race that instantly became one of the greatest ever run.

The achievement sent shockwaves throughout the track and field community. While Tharp arrived in Eugene as one of the leading contenders for the NCAA title, few envisioned a performance capable of redefining the limits of the event. Instead, the American hurdler delivered a race for the ages, combining flawless technique, explosive speed, and impeccable rhythm from the first hurdle to the finish line.

The historic run not only secured his place in athletics history but also transformed the outlook of the championship. With the world record now in his possession, Tharp advances to the final as the overwhelming favorite, carrying momentum that could make an already unforgettable weekend even more extraordinary.

For years, the 12.80 barrier appeared untouchable. On a stunning day at the NCAA Championships, Ja’Kobe Tharp proved otherwise, producing the kind of performance that reminds fans why sport remains so unpredictable. In a matter of seconds, he turned a routine qualifying round into a landmark moment that will be remembered for generations.

The world record no one saw coming is now a reality—and Ja’Kobe Tharp is the man who changed history.

A new chapter in middle-distance running may have begun in Oslo after American teenager Cooper Lutkenhaus produced one of the most remarkable performances of the season, narrowly defeating reigning Olympic champion Emmanuel Wanyonyi in a thrilling men's 800m contest at the Diamond League meeting.

The 17-year-old sensation shocked a world-class field by crossing the finish line first in a race that came down to the smallest of margins. After an intense battle over the final metres, Lutkenhaus held off Wanyonyi by just one hundredth of a second, producing a dramatic finish that left the packed stadium in disbelief.

From the opening lap, the pace was relentless as the leading contenders positioned themselves for a fierce showdown. As the athletes entered the home straight, Wanyonyi appeared poised to unleash his trademark finishing kick. However, Lutkenhaus refused to be intimidated, matching the Olympic champion stride for stride before producing a perfectly timed lean at the line to secure a historic victory.

The result marks a breakthrough moment for the young American, who continues to establish himself as one of the brightest talents in global athletics. Defeating an Olympic champion at a Diamond League event is a feat many athletes spend entire careers pursuing, yet Lutkenhaus achieved it before reaching adulthood.

For Wanyonyi, the narrow defeat does little to diminish his status as one of the world's premier 800m runners. The Kenyan once again demonstrated his exceptional class and competitiveness, pushing the race to a world-class standard and forcing his young rival to deliver the performance of a lifetime.

Beyond the result itself, the race offered a glimpse into what could become one of the sport's most exciting rivalries in the years ahead. With established stars and emerging talents now pushing each other to new heights, the men's 800m continues to evolve into one of athletics' most captivating events.

On a memorable night in Oslo, the spotlight belonged to Cooper Lutkenhaus. At just 17 years old, he stood toe-to-toe with an Olympic champion and emerged victorious, announcing himself to the athletics world in spectacular fashion.