Exercise or rest? This is how you know if you can exercise when you're sick

Getting sick is, besides injuries, the biggest obstacle for athletes. Should you rest completely, or can you still do a lighter workout? How does exercise really affect the immune system, and what are the risks if you train while your body is fighting an infection?

There are many myths and misconceptions about training when sick, so today we thought we’d provide clear guidelines. In this post, we will go through what happens in the body when you train while sick, how to determine whether you should rest or move, and how to return to training in the best way.

How does training affect the immune system?

Training and the immune system have a complex relationship. Regular physical activity can strengthen the immune system in the long term by improving circulation and stimulating the body’s defense cells. However, when you train intensely—especially during longer endurance sessions or heavy strength workouts—you create a temporary strain on the body.

Studies show that after an intense workout, the body enters a period of weakened immunity, sometimes called the "open window." For the next 3-72 hours, you are more susceptible to infections because your immune system is busy repairing the body after training. This is one of the reasons many athletes often catch colds after intense training periods or competitions.

When sick, the immune system is already overloaded, and putting additional physical stress on the body can have negative consequences. In the worst case, it can lead to serious complications such as myocarditis (inflammation of the heart muscle).

Conclusion: Regular training strengthens the immune system in the long run, but training while already sick can make you sicker and prolong your recovery.

Cold or Flu – What’s the Difference?

To decide whether you can train or not, it’s important to understand the difference between a common cold and the flu.

Cold
A cold typically develops gradually and mostly affects the upper respiratory system. You may experience a sore throat, a stuffy or runny nose, and mild coughing. Sometimes mild fever and slight body aches occur, but cold symptoms are generally milder than flu symptoms. With a mild cold, it may be possible to train, but it completely depends on your symptoms.

Flu
The flu, on the other hand, is a completely different story. It often strikes suddenly and can cause high fever, chills, extreme fatigue, headaches, muscle aches, and dry cough. The flu is much more exhausting than a cold and can lead to serious complications.

If you have the flu, you should always rest until you feel fully recovered.

Can You Train When Sick? A Symptom Guide

There’s a simple rule to follow: "Above the neck." If your symptoms are mild and limited to your nose and throat, light exercise may be okay. If your symptoms extend below the neck, such as coughing, chest pain, or fever, you should avoid training altogether.

When to Rest Completely
If you experience fever, muscle and joint pain, severe headaches, fatigue, or deep coughing, it’s best to let your body rest. These symptoms indicate that your body is fighting an infection, and putting additional strain on it with exercise could worsen the illness. Moreover, the risk of heart complications increases, which could lead to an even longer recovery period.

When Light Exercise May Be Okay
If you only have mild sore throat, nasal congestion, or a mild cold, low-intensity exercise may be an option. However, it’s still important to listen to your body—if you feel unusually tired or weak, it might be wise to take an extra rest day.

What Type of Exercise Can You Do When Sick?

If you decide to move despite having a mild cold, it’s important to choose the right type of exercise.

• Light Walks: A gentle walk can increase circulation and help with nasal congestion. Fresh air can also provide a mental boost.
• Yin Yoga or Stretching: If you feel stiff but don’t have a fever, gentle mobility training can help keep your body moving without overburdening it.
• Low-Intensity Strength Training: Light weights or bodyweight exercises can work if you feel relatively okay, but avoid high heart rates and exertion.
• Avoid high-intensity exercise like HIIT, heavy lifting, and intense cardio until you’re completely well.

Common Mistakes and Myths About Training When Sick

"You can sweat out the illness"

This is one of the most persistent myths. Sweating more does not help the body fight the virus faster. On the contrary, dehydration can make you even more tired and prolong your illness.

"If I don’t have a fever, I can train as usual"
Fever is a clear signal that your body needs rest, but the absence of fever doesn’t mean you’re ready to train. Extreme fatigue, muscle pain, and deep coughing can also be signs that your body needs recovery.

"Vitamin C and Vitamin D cure the cold"
Research shows that high doses of vitamins don’t shorten the duration of illness, but regular intake of Vitamin D can reduce the risk of getting sick during the winter months.

"I have to make up for missed training after being sick"
Trying to make up for missed workouts by training extra hard immediately after being sick is a bad idea. Your body needs time to recover, and pushing too hard can increase the risk of injury and overtraining.

How Can You Speed Up Your Recovery?

To get better faster, the most important thing is to give your body the right conditions. Make sure to get enough sleep and rest, eat a nutrient-rich diet full of fruits and vegetables, and stay well hydrated. Drink, drink, drink!

Studies also show that zinc supplements can shorten the duration of illness if taken within 24 hours of the onset of symptoms. This is one of the few supplements with scientific backing when it comes to cold recovery.

Summary: When Is It Okay to Train?

If you have fever, muscle aches, or extreme fatigue, you should always rest. If you have a mild cold without fever, low-intensity exercise may be an option—but rest might sometimes be a better strategy for faster recovery. Listen to your body and let it set the pace.

Training strengthens the immune system in the long run, but when you’re sick, it can do more harm than good. Take care of your body, and you’ll come back stronger and faster!