What Is Runner’s Flu, and Why Do You Get Sick After a Marathon?

After months of intense training and finally crossing the finish line, you find yourself sick after running a marathon. Whether it’s stomach upset or flu-like symptoms, it’s normal to feel sick after a race — and it’s more common than you might think.

While there can be many factors at play, experts say that what goes on inside our bodies during a marathon may contribute to illness. Here’s a closer look at why that might happen, and how to prevent and treat the dreaded marathon flu, also known as runner’s flu.

3 Reasons You Might Feel Sick After a Marathon

Erica Coviello, a Road Runners Club of America (RRCA)–certified running coach and an American Council on Exercise (ACE)–certified personal trainer, has seen her athletes get wiped out post-marathon. “It could be a combination of things,” she says. “A marathon totally depletes you, and there are a few reasons why you might feel awful.”

1. Gastrointestinal Issues

Upset stomach and the need for urgent bathroom breaks are common among marathon runners.

“When you hear about people needing bathroom stops and their gut being totally destroyed, it’s usually because when you’re running the marathon, your [physical] stress levels go high,” Coviello says. “It puts you into that flight-or-fight response mode, and in doing that, your blood flow changes and your body is like, ‘What the heck is going on?’ It reroutes the blood to essential organs and away from your gut.”

This is the leading theory behind why you might feel nauseous or like you are going to have a No. 2 emergency mid-race. But, according to Coviello, this can actually continue on for several days after your marathon.

2. ‘Post-Marathon Depression’

Perhaps just as common as the marathon flu would be the marathon blues, which is a feeling of loss and sadness following the race.

“You spend so much time and so much of your schedule regimented toward that one event,” Coviello says. “Whether you completely crush your goals or you have completely failed, that’s the climax. Then you’ve got a day or two of celebration, and then many people wonder what they are supposed to do now.”

That void can feel like depression, and if you are diagnosed with a mental health condition like depression or anxiety, it can exacerbate symptoms.

“There is a real physical manifestation of that,” Coviello says. “And if that’s why you feel crappy, you have to treat it like a real thing to recover.”

3. You Have an Infection

For many reasons that we’ll get into below, the marathon flu is usually just that: a real flu, cold, sinus infection, or other virus that requires rest and care.

What Runner’s Flu Really Feels Like

• Runny nose
• Cough
• Fever or chills
• Body aches
• Headache
• General malaise

But see your doctor right away if you feel like you’re dealing with more than cold or flu symptoms and you have:

• Vomiting
• Dark-colored urine
• Extreme fatigue
• Severe muscle cramps

Causes of Marathon Flu

Ian McKeag, MD, an associate professor of family and community medicine at the University of Alabama at Birmingham’s School of Medicine, explains how strenuous exercise (like running 26.2 miles in one go) affects the body.

“When you’re running a marathon — or even a half-marathon — you’re putting yourself through a lot of physical demands,” he says. “You’re burning all sorts of energy, expending glucose and glycogen, and the body is only really equipped for about an hour before it starts to pull from other sources. And as it starts to pull from some of those sources, that puts just more demand on the body as a whole.”

Immunity dips, leaving us at risk of getting sick. There’s also a lot going on with our hormones during training that can cause issues.

“When you are training for a marathon and persistently putting the body in that kind of stress, things like cortisol levels change,” Dr. McKeag says. “That’s a stress hormone that can have a negative impact on our immune response. That’s one reason the immune system isn’t as strong after running longer distances.”

Another big factor is exposure to germs.

“On the day of the race, runners are put into a corral packed in with other runners, and there is a bottleneck for the first few miles of the race — you’re going to be exposed to a big, diverse group of people,” McKeag says.

Coviello agrees, noting that you can also be exposed to infectious bugs while traveling to a race.

“And in the race, when your immune system is already compromised, you’ve got all these germs that are around you,” she says. “Then you’ve got people that are throwing snot rockets all over the place. Then at the end of the race, you’ve got all these snot- and sweat-filled bodies, and everybody's hugging each other.”

How to Prevent Runner's Flu

After a marathon, you’re going to be rundown, and runners pass germs to each other all the time — even more so in big races. But even if the odds are stacked against your health on race day, McKeag and Coviello say not to despair. There are things you can do before the marathon to improve your chances of staying healthy.

“It’s the other stuff that [runners] are not doing to take care of themselves throughout training, in my opinion, that is really putting them at the greatest risk,” McKeag says. “Refueling, rehydrating, getting restful sleep, and controlling stress — especially after any run greater than an hour.”   “ ”

Nailing your sleep and food intake are two things that can help make sure you’re feeling your best before, during, and after a marathon. If you want to be proactive, focus on micronutrients as well as macronutrients in your diet, Coviello says.

“You have to find your good ratio [of macros] and make sure you’re getting enough calories from carbs, protein, and fat,” she explains. “But your immune system also relies on the right micronutrients or vitamins or minerals, especially things like vitamin C, before the race so that your body isn’t playing catch-up.”

How to Treat Runner’s Flu Post-Marathon

When it comes to recovering from the marathon flu, there is no magic pill or potion. Patience and self-care are the name of the game. McKeag says the best remedies are the three golden R’s: refuel, rehydrate, and recover.

“Get some good sleep, get some good food in you, and always, always stay well-hydrated,” he says.

Your body will need rest, so don't get hung up on missing miles, especially if you have a fever.

“If your temperature is anywhere above 100.4 degrees Fahrenheit, you should not exercise — no ifs, ands, or buts,” McKeag says. “And if you just feel like crap, take time off training.”

Even if you’re not sick, Coviello advises taking a full two weeks off from running after a marathon to give your immune system, muscles, heart, lungs, stomach, and brain a chance to fully recover.

The Takeaway

• Runner’s flu can occur after a marathon and leave you feeling rundown with symptoms like sore throat, runny nose, fatigue, and fever or chills.
• A dip in immunity due to strenuous exercise combined with exposure to germs from a large number of runners on race day can raise your risk of marathon flu.
• Taking precautions while training, including prioritizing nutrition, sleep, and hydration, can help prevent sickness after running a marathon.