I’m Sick, Should I Exercise?

Key Takeaways:

• Exercise boosts your immune system in the short term by up-regulating key immune cells.

• Regular moderate exercise reduces likelihood of infection, severity of symptoms, and length of illness in the long term.

• Overtraining without rest can impair your immune system.

---

We are in the “viral” season so chances are you or someone you know is sick. Illnesses can throw your training and general fitness off track. It can derail your routines and destroy your motivation. You may be wondering: should I continue to exercise when I’m sick? 

Physical therapists are consulted by physicians in the hospital to help the sickest patients exercise. Sometimes all they can do is walk down the hall, do some exercises in bed, or move to sit in the chair next to their bed. Why don’t these patients just rest?

Aside from preventing the risks associated with deconditioning and immobility in the hospital (pneumonias, bed sores, etc), physicians and physical therapists know that exercise can also boost your immune system and help you get better, faster. The old belief that you should rest when you are sick has been debunked. Here is the science behind why exercise is such an effective immune booster:

⸻

🏃‍♂️ Immediate Immune Benefits (Within Minutes to Hours)

1. Increased circulation of immune cells

During and immediately after exercise, your body mobilizes immune cells into the bloodstream:

Key cellular changes:

• Neutrophils increase rapidly.

• Natural Killer (NK) cells surge 2–5× above resting levels; these cells are powerful antiviral and antitumor defenders.

• Cytotoxic T cells (CD8+) increase in circulation.

• Monocytes shift into the bloodstream and become more activated.

This surge helps the immune system detect pathogens or damaged cells more effectively.

2. Enhanced immune cell trafficking

Exercise causes:

• Faster immune surveillance in the lungs, gut, and peripheral tissues.

• Redistribution of immune cells to areas where pathogens most commonly enter.

This improves early detection and clearance of viruses and bacteria.

3. Temporary reduction in inflammation

Right after moderate exercise:

• Anti-inflammatory cytokines (IL-10, IL-1RA) rise.

• Pro-inflammatory cytokines (TNF-α, IL-6 from adipose tissue) temporarily decrease.

• Contracting muscle produces myokine IL-6, which acts anti-inflammatory in the context of exercise (very different from chronic inflammatory IL-6).

This creates a healthier inflammatory environment.

4. Improved stress-hormone balance

Short bouts of exercise:

• Increase epinephrine and norepinephrine, which stimulate immune cell activity.

• Reduce cortisol levels afterward, helping regulate immune function.

⸻

🔁 Long-Term Immune Benefits (Weeks to Years)

When exercise is consistent (≥150 min/week moderate intensity), the immune system becomes more resilient.

1. Lower chronic inflammation

Regular exercise leads to:

• Lower resting IL-6, TNF-α, and CRP levels.

• Reduced inflammation from visceral fat as body composition improves.

This slows down low-grade chronic inflammation associated with aging.

2. Stronger adaptive immunity

Over time, moderate exercise:

• Enhances antibody responses (e.g., stronger vaccine responses in adults and older populations).

• Improves T-cell activation and function.

• Slows age-related decline in naïve T-cell production.

This helps your immune system adapt better to new pathogens.

3. Healthier immune cell environment

Consistent exercise drives long-term changes such as:

• Increased NK cell cytotoxicity (better cancer surveillance).

• Improved phagocytic activity of neutrophils and macrophages.

• Better regulation of regulatory T cells (Tregs), supporting immune balance.

4. Enhanced mitochondrial health

Muscle and immune cell mitochondria become more efficient:

• Better ATP availability for immune responses.

• Lower oxidative stress.

• Improved resilience of immune cells under metabolic stress.

5. Lower infection risk

Studies show that people who exercise regularly have:

• 30–50% fewer upper respiratory tract infections annually.

• Faster recovery when infections do occur.

This comes from a combination of better surveillance, lower inflammation, and better immune regulation.

⸻

⚠️ A Note on Overtraining

Moderate exercise boosts immunity, but:

• Excessive endurance training without recovery can temporarily depress immune function.

• This manifests as reduced NK cell activity, lower IgA in saliva, and higher cortisol.

Balance and recovery remain essential.

⸻

🛑 When to Avoid Exercise

If you have a fever or if a physician otherwise advised you not to exercise, then you should rest. Always follow the advice of your doctor as they know your medical history and any risks that may pertain to you. 

Also, if you train or exercise with a group or team, consider distancing, wearing a mask, or exercising independently until you are well.

⸻

✅ Summary

If you are fever-free and your doctor has not advised you rest, exercise is excellent for recovering from your cold and will help the next one be less severe, shorter, or you may avoid it altogether.

Immediate effects (within hours)

• Surge in NK cells, neutrophils, monocytes, and T cells

• Enhanced immune surveillance

• Reduced inflammation through exercise-induced myokines

• Improved stress hormone regulation

Long-term effects (weeks–years)

• Lower chronic inflammation

• Stronger adaptive immunity

• Increased NK cell and macrophage function

• Healthier mitochondria in immune cells

• Lower rates of infection