You have a cold, a race in three weeks, and a nagging voice telling you that missing a run will ruin everything. So should you train through it, or rest? Here is the honest answer.
Every autumn and winter I get the same question from athletes. "I feel rough, do I still run?" Close behind it is the worry that all their hard training is somehow wrecking their immune system and leaving them ill all the time.
The science here has shifted, and a lot of the old fear was overblown. Let me walk you through what training actually does to your immunity, what really makes you more likely to get ill, and a simple rule for deciding whether to lace up or stay on the sofa.
For years the story was that hard exercise opens a dangerous "window" where you are wide open to infection. That idea is now contested. Many researchers think what looks like immune suppression after a hard effort is actually your immune cells redeploying around the body to do their job, not switching off.
The bigger drivers of getting ill are things you can manage: under-fuelling, poor sleep, too much training load too soon, life stress, and simple exposure to bugs. So the goal is not to fear your training. It is to support your body around it.
The "Open Window" Story, and Why It's Debated
You may have heard that after a long, hard session your immune system dips for a few hours to days, leaving an open window for infection. It came from older research showing changes in immune cells in the blood after marathons and similar efforts.
- The drop in immune cells in the blood after exercise does not mean they have vanished. More likely they have moved into tissues like the lungs and gut to patrol for threats. That is your immune system working, not failing.
- Reports of athletes getting ill after big events are real, but the cause is tangled up with travel, crowds, broken sleep, under-fuelling and stress, not the exercise alone.
- Plenty of the "infections" reported after endurance events turn out not to be infections at all, but inflammation or irritation of the airways.
So the picture is not "hard running makes you sick". It is "doing too much, on too little fuel and sleep, while run down and surrounded by bugs, makes you more likely to get sick". That is a much more useful thing to know, because every part of it is in your hands.
What Actually Raises Your Risk of Getting Ill
These are the real levers, and most of them have nothing to do with how fast you ran.
Under-fuelling. Training hard while not eating enough, especially too little carbohydrate, is one of the clearest ways to leave yourself run down and prone to illness. This is common in women and it matters more than almost anything else here.
Poor sleep. Short or broken sleep is strongly linked to catching more coughs and colds. It is probably the single biggest everyday factor.
Ramping load too fast. A sudden jump in training, or stacking hard week on hard week with no easier weeks, leaves you frayed.
Life stress. Work, caring, poor recovery and chronic stress all tax the same system.
Exposure. Sometimes you just catch what is going round. Busy trains, offices, kids bringing bugs home. Not every cold is a training error.
Notice that "I did a hard interval session" is not on that list as a standalone cause. Hard sessions are fine when they sit on top of enough food, sleep and recovery.
Should You Run With a Cold? The Neck Check
This is the practical bit everyone wants. The widely used rule of thumb is the neck check. It is not a medical law, but it is a sensible guide.
- Symptoms above the neck only, like a runny or blocked nose, sneezing, a mild sore throat, and you feel otherwise okay? A gentle, easy run or a shorter session is usually fine. Keep it light.
- Symptoms below the neck, like a chesty cough, tight or heavy chest, aching muscles, or a fever? Rest. Do not train. This is where pushing on can turn a minor illness into something that keeps you out for weeks.
- A fever is a hard stop. Never train with a temperature. Exercising with a fever is genuinely risky for your heart. Rest completely until it has gone and you feel well.
When in doubt, miss the session. One easy run is almost never worth it, and you lose very little fitness from a few days off. Pushing through a real illness can cost you weeks. The maths is not close.
Three Mistakes I See Every Winter
Chesty, feverish, aching, and still grinding out the plan because you are scared to miss it. This is how a cold becomes a two-week write-off, or worse.
Fix: below-the-neck symptoms or a fever means rest. Full stop.
The opposite problem. A mild head cold does not mean two weeks on the sofa. Gentle movement is usually fine and often feels better than doing nothing.
Fix: above-the-neck only and feeling okay? Keep it easy, but keep moving.
No pill, powder or shot beats eating enough, sleeping well and managing your load. Most immune supplements do very little, and some are a waste of money.
Fix: sort sleep, fuel and load first. Those are your real immune support.
Ten questions, two minutes. Tells you whether to push this week, hold steady, or back off.
How to Support Your Immunity Day to Day
If you want to get ill less often, these are the things that genuinely help, and none of them come in a bottle.
- Eat enough, especially carbohydrate. Fuelling your training properly is immune support. Chronic under-eating is the opposite.
- Protect your sleep. The most powerful recovery and immune tool you have, and it is free.
- Get your protein and key micronutrients in. Enough protein, plus iron and vitamin D sorted, which are common gaps in women.
- Respect easy weeks. Build in lighter weeks and recovery. Do not stack hard on hard.
- Basic hygiene in busy seasons. Wash your hands, and give yourself permission to rest when a bug is going round.
Coming Back After Being Ill
When you do get knocked out by something, come back gradually rather than straight into your hardest session. A good guide is to wait until your symptoms have gone, start with easy, shorter runs, and judge each one by feel. If an easy run feels much harder than it should, or your heart rate is oddly high for the effort, back off and give it another day. After a fever or anything that hit you hard, be especially patient. Rushing back is how people relapse or pick up a second bug on a tired body. You will not lose meaningful fitness from a cautious week. You can lose a lot from forcing it and ending up ill again.
Does Your Cycle Affect How Run Down You Feel?
Lots of women notice they feel more fragile, more tired and more likely to catch something in the days before their period. There is a sensible explanation. In the second half of your cycle, after ovulation, your body temperature runs slightly higher, your resting heart rate can creep up, and inflammation shifts. None of that means you are ill, but it can leave you feeling more run down and make a hard session feel harder than usual.
The practical point is not to panic or to stop training, but to read your body. If you feel wrecked and vulnerable premenstrually, that is a reasonable week to be a little more generous with rest, keep your fuelling up, and not force a brutal session just because the plan says so. Work with your cycle rather than ignoring it. I cover this properly in my piece on training with your cycle.
Big Events and the Days Around Them
If there is a time to be careful, it is the week around a big race, especially one with travel involved. This is where a lot of post-event illness actually comes from, and almost none of it is the running itself.
- Protect your sleep in the build-up. Travel and nerves wreck it, and poor sleep is your biggest risk factor.
- Keep fuelling properly. Do not combine tapering with under-eating. Your body needs fuel to stay robust.
- Be sensible in crowds and on planes. Wash your hands and do not share bottles. Expo halls and packed start pens are germ soup.
- Go gently on the celebratory drinking straight after. A big night on a depleted, tired body does your immune system no favours.
What About Vitamin C, Zinc and the Immune Shots?
Every winter the shelves fill with immune-boosting pills and powders. The honest truth is that most of them do very little for a well-fed person. Vitamin C does not prevent colds in general, though it may slightly shorten one, and megadosing is pointless and can upset your stomach. Zinc has some evidence for shortening a cold if taken very early, but it is modest. The various echinacea and "immunity" shots are mostly hopeful marketing. The one genuinely worth attention for many women in the UK, especially over winter, is vitamin D, because low levels are common here and are linked to immune function. If you are going to spend money on anything, a sensible vitamin D supplement over the darker months beats a cupboard full of immune shots. Beyond that, your food, sleep and training do far more than any pill on the shelf.
Early Warning Signs Worth Heeding
Your body usually whispers before it shouts. A few signs are worth taking seriously as a cue to ease off before a niggle becomes a full illness. A resting heart rate that is noticeably higher than normal for a few mornings. Sleep that suddenly goes to pieces. A session that feels far harder than it should for no clear reason. A scratchy throat that will not settle, or just a heavy, run-down feeling that lingers. One of these on its own is often nothing. Several together are your body asking for a lighter few days. The athletes who stay healthiest are not the ones who never feel tired, they are the ones who notice these early signals and back off for a day or two rather than grinding on and getting properly ill. Easing off early almost always costs you less than pushing through.
The Short Version
Regular sensible training supports your immune system. The old "open window" fear is contested.
The real drivers of getting ill are under-fuelling, poor sleep, ramping load too fast, stress and plain exposure.
Use the neck check. Above the neck and feeling okay, an easy run is usually fine. Below the neck or any fever, rest.
A fever is a hard stop. Never train with a temperature.
When unsure, miss the session. Days off cost little, pushing through real illness can cost weeks.
Support immunity with food, sleep, protein and load management, not supplements.
Train sensibly, fuel and sleep like they matter, and stop fearing that your running is making you ill. It almost certainly is not. Look after the basics and your body will look after you. Find your glow.
This article is general information, not medical advice. If you have a fever, a chest infection, breathlessness, or symptoms that persist or worsen, do not train and see your GP. Always stop exercising and seek advice if you feel unwell during activity.
- You panic about missing a run even when you feel rough
- You seem to pick up every bug going and wonder if your training is to blame
- You never quite know whether to push through or rest
- You want a coach who adjusts your plan around real life, not a rigid schedule
If you ticked two or more, this is exactly the kind of thing I help with.
Apply to work with me →If you want a plan that flexes when life and illness get in the way, instead of a rigid schedule that makes you feel guilty, that is what I do.
Female-specific endurance coaching, built around your real life, your recovery and your body.
The application takes two minutes. I read every one myself. I will WhatsApp you within 24 hours on weekdays.
Apply to work with me →Frankie x