If you feel like you catch everything going and take longer than everyone else to shake it off, the useful news is that this is rarely bad luck and almost never a broken immune system. It is usually two or three specific, measurable, changeable things. This is what they are, in the order the evidence says they matter, and what to do about each one.
On this page
First, is it actually too often?
Worth settling before anything else, because a lot of people carrying this worry are within normal range and spending energy on it.
Two to four colds a year is typical for an adult. More if you have young children, teach, nurse, or deal with the public. Three colds across a winter is not evidence of anything wrong.
What is worth paying attention to is not the count but the pattern: a clear change from your own baseline, infections that drag on for weeks rather than days, the same infection returning repeatedly, or anything needing repeated courses of antibiotics. That last group is a doctor's appointment rather than an article, and there is a section at the end on exactly when.
If you are in the normal range and simply want fewer of them, everything below applies.
What is working against you
This half comes first deliberately. Removing something that is suppressing you tends to be worth more than adding something on top, and these are the levers almost nobody checks.
Short sleep, and the threshold is sharp
This is the strongest single finding in the whole field, and it comes from a study design that is hard to argue with.
Researchers measured the sleep of 164 healthy adults for a week using wrist actigraphy, so actual measured sleep rather than what people said they got. Then they put them in quarantine, dripped rhinovirus into their noses, and watched who got a cold.
4×
People sleeping under six hours a night were roughly four times more likely to develop a cold after direct viral exposure than those sleeping more than seven.
Prather et al, Sleep, 2015. 164 adults, actigraphy-measured sleep, experimental rhinovirus challenge.
Two details make this more useful than the headline. The effect held independently of pre-existing antibody levels, age, season, body mass index, psychological factors and general health habits, so it is not simply that people who sleep badly also live badly.
And the threshold sits in a specific place. Under five hours and five-to-six hours both carried roughly four times the risk. Six to seven hours was not significantly different from sleeping more than seven. The cliff is below six, not below eight.
It was also duration that mattered, not how broken the night was. Sleep fragmentation showed no relationship. If you are getting seven hours in a somewhat disturbed form, that is a far better position than six clean ones.
The confidence intervals on those risk figures are wide, which is worth knowing. The direction and the size are clear enough to act on. The precision is not.
Long-running stress, but not the kind you are thinking of
The distinction here is the whole point, and almost every article about stress and immunity flattens it.
In a study of 276 people who were interviewed about life stressors and then deliberately exposed to cold viruses, severe acute stressful events lasting under a month were not associated with getting sick at all. Severe chronic stressors lasting a month or longer were associated with a substantial increase in risk.
More specifically, the effect was driven by two things: being unemployed or underemployed, and enduring difficulty with family or friends. Not deadlines. Not a bad week. Situations with no visible end.
The same research group later described a plausible mechanism: prolonged stress appears to make immune cells less responsive to cortisol's signal to stand down, so the inflammatory response stops being properly regulated.
The practical reading is unglamorous but genuinely freeing. A stressful fortnight is not damaging your immune system. A two-year situation you cannot see the end of is a health variable, and treating it as one is reasonable.
Doing very little, most weeks
Covered properly in the next section, because the same evidence reads better as something to add than as something to blame yourself for.
The things you already suspect
Alcohol, smoking and vaping all impair the defences of your airways and the immune cells behind them. None of that is controversial and none of it needs a citation to be believed. What is worth saying is that these are dose questions, not moral ones. Fewer drinks in a week is a real change even if the number is not zero.
Undereating deserves a mention because it is rarely on anyone's list. Producing immune cells and antibodies takes protein and energy. Someone eating very little, cutting hard for a goal, or simply skipping meals under pressure is asking their body to mount a defence on a restricted budget.
What is working against you
In rough order of how much difference each one makes, and how confident the evidence is.
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Under six hours of sleep
Roughly four times the risk after direct viral exposure. Measured, not self-reported.
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A stressor lasting months
Chronic only. Short sharp stress showed no effect in the same study.
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Almost no physical activity
The least active adults had roughly twice as many days unwell as the most active.
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A winter with no sun
At New Zealand's latitude your skin makes very little vitamin D between May and August.
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Regular alcohol, smoking, vaping
All impair airway defences. A dose question rather than an all-or-nothing one.
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Eating too little
Immune cells and antibodies are built from protein and energy you have to supply.
Most
people have one or two of these, not all six
Which is the encouraging part. Finding your one or two is a far better position than deciding you simply have a weak immune system.
How to read this. Ordered by a combination of effect size and how strong the study design was. The top three all come from research where people were either deliberately exposed to a virus under controlled conditions or tracked prospectively through a full cold season.
What actually helps
Get above six hours, and protect it like an appointment
Given the evidence above, this is the highest-value change available to most people, and it is free.
The practical version is not a sleep-hygiene lecture. It is one decision: work out what time you need to be in bed to clear seven hours, and treat that time the way you would treat a meeting you cannot move. Most people who sleep six hours are not struggling to sleep. They are going to bed too late.
If the problem is the other kind, where you are in bed for eight hours and getting far less, that is a different job and we have covered it properly in our guide to sleep.
Move most days, at a level you would call easy
This one is more powerful than its reputation and the numbers are worth seeing.
A study followed 1,002 adults aged 18 to 85 through 12 weeks of autumn and winter, recording every respiratory symptom. Those doing aerobic exercise five or more days a week had 43 per cent fewer days unwell than those managing one day or fewer. Symptom severity was 32 to 41 per cent lower, and the gap between the fittest and least fit third of the group was similar.
Two things shape how to read it. It is observational rather than a trial, so it shows a strong association rather than proving cause. And fitness was self-rated. It was adjusted for seven confounding factors, and the size of the difference is large enough to be worth acting on regardless.
There is a real ceiling, though, and it is the reason this section says "at a level you would call easy". Endurance athletes show increased infection risk during heavy training blocks and for one to two weeks after race events, with measurable transient suppression of several immune functions after very hard efforts. Moderate and regular is the protective pattern. Occasional and brutal is not.
For most people reading this, the relevant end is the bottom one. Walking most days is the intervention.
Deal with the situation, not the symptom
If the stress in your life is a genuinely chronic situation rather than a busy period, then the intervention is the situation. That may mean a conversation you have been avoiding, professional help, or a change you have been putting off for a year.
Nothing in a tub competes with resolving a two-year problem, and it would be dishonest to pretend otherwise.
Eat enough, and eat enough different things
Two separate points that get merged.
Enough total food and enough protein. Antibodies are proteins. Immune cells divide rapidly and that costs energy. Chronic under-eating is genuinely immunosuppressive and it is common among people who would never describe themselves as under-eating.
Enough variety. Most of your immune tissue sits in and around your gut, and the bacteria there are shaped by the range of plants you eat. That is a slower lever than sleep and a real one, and our guide to improving your gut health goes through what shifts it.
Sunlight, and why New Zealand is a special case
Your skin makes vitamin D from UVB, and at our latitude there is very little usable UVB through the middle of winter no matter how bright the day looks. Anyone spending winter indoors, covering up, or living further south is likely to drift downwards through the season.
Fifteen minutes of midday sun on arms and face through the warmer months does most of the work. Through winter, sun exposure alone often does not, which is the honest argument for supplementing that specific nutrient in that specific season.
Where supplements genuinely fit
This section covers what has actual human evidence behind it, whether or not we sell it. Several of the strongest entries below are things you would buy somewhere else, and one of them is the opposite of what we make.
Two things worth knowing before the table. Almost all of this research is on people who were already taking something daily before they got sick, rather than reaching for it once symptoms started. And these are group averages: a 10 per cent shorter cold is a real effect that you personally may not notice.
| What | What the human research shows | How to actually use it |
|---|---|---|
| Vitamin C | Across 10 randomised double-blind trials using at least 1g a day, average cold severity was 15 per cent lower. Regular intake also shortened colds by about 8 per cent in adults and 14 per cent in children. In marathon runners, skiers and soldiers on subarctic exercises, it roughly halved the rate of colds. |
Daily and ongoing, at 1g or more. Its effect comes from already being on board when you meet a virus, so it belongs in your routine rather than in the medicine cupboard. Much above 1g tends to cause digestive upset without adding benefit. |
| Vitamin D | Pooled randomised trials show a reduction in respiratory infection risk, and separately about 6 per cent shorter symptoms. Steady daily dosing worked; occasional large doses did not. |
Daily through a New Zealand winter, at 400 to 1,000 IU, which is the range where the benefit showed up. The one nutrient here with a genuine seasonal case at our latitude. |
| Zinc | In pooled trials it did not stop people catching infections, but it shortened symptoms by around 47 per cent, which is the largest duration effect of anything in this table. Separately, high-dose zinc acetate lozenges have their own evidence for shortening colds. |
A daily nutritional dose keeps you topped up, which is what the duration research was based on. The lozenge research used 75 to 100mg a day dissolved in the mouth at symptom onset, which is a short-term approach and a different thing entirely. |
| Probiotics | The strongest prevention evidence in this table. Across 23 trials and 6,950 people, 24 per cent fewer people caught a respiratory infection, episodes were about 1.2 days shorter, and 42 per cent fewer people needed prescribed antibiotics. |
Daily, and give it months rather than weeks, which is how the trials ran. Strain and dose vary enormously between products, so choose one that names both rather than one that only advertises billions of organisms. |
| Elderberry | A meta-analysis of randomised trials found elderberry substantially reduced upper respiratory symptoms, with a large average effect. The caveat is size: 180 participants in total. |
Used at the onset of symptoms in most of those trials, which makes it one of the few things here with a case for reaching for it when you feel something starting. |
| Selenium | Essential for normal immune function, and deficiency measurably impairs it. New Zealand soils are naturally low in selenium, so local dietary intake tends to run lower than in most countries. |
A modest daily amount. This is about not being short of it rather than about taking extra, and the New Zealand soil situation is a real local reason to pay attention. |
| Beta-glucan | Yeast beta-glucan is recognised by receptors on your innate immune cells, including one called Dectin-1, and features in research on whether those cells can be primed to respond faster. A different mechanism from every vitamin and mineral above. |
Daily, as part of a routine rather than reactively. Because it works through a different pathway from the vitamins and minerals above, it adds something they do not, rather than duplicating them. |
| Garlic | One good-quality trial of 146 people taking a daily allicin supplement for 12 weeks found 24 colds versus 65 on placebo, and 111 days of illness versus 366. Recovery time was similar once a cold started. |
Promising rather than proven, and Cochrane is clear that one trial is not enough. Eating it costs nothing, so this is a reasonable thing to do more of while the evidence catches up. |
How to read this. Ordered roughly by how much good evidence sits behind each one rather than by how well known it is. Probiotics and vitamin C have the largest bodies of trial evidence; garlic and elderberry have the smallest. All of these are research on nutrients and extracts, not on any particular product.
The one thing worth changing about how you take vitamin C
Vitamin C does real work. It is concentrated in your white blood cells, supports the barrier function of the tissue lining your airways, and is involved in how neutrophils move to an infection and clear it. Low vitamin C status measurably impairs all of that, and infection itself drains your stores further.
Where it gets misused is timing. The trials that found shorter, milder colds were all testing people who had been taking it regularly before they got ill. Trials that started it only after symptoms appeared have not shown a consistent benefit.
So the change is not whether to take it, it is when. Vitamin C is a routine, not a rescue. If yours lives in the cupboard for emergencies, moving it to the bench next to the kettle is the single adjustment that makes it work the way the research says it can.
The vitamin D disagreement, reported rather than resolved
The two largest analyses do not fully agree, and picking the more flattering one would be easy.
The 2017 individual-participant analysis of 25 trials found a clear overall benefit, and a much larger one among people who were severely deficient at the start. That deficiency finding is widely quoted, including by supplement companies.
The 2021 update, covering 46 trials and 75,541 people, found a smaller overall benefit and reported no significant effect for any group defined by starting vitamin D level. The authors describe the reduction as small.
Same overlapping research teams, more data, a weaker and less targeted result. What survives both is narrow and still worth acting on: a modest reduction in respiratory infection risk, with daily dosing clearly outperforming occasional large doses, at daily amounts in the hundreds rather than thousands of IU. Which is a reasonable basis for taking a moderate amount through winter, and not a basis for expecting it to stop you getting sick.
When it is not lifestyle
Most frequent illness is explained by the things above. A small amount is not, and the difference is in the pattern rather than the frequency.
A month that would actually change something
Everything above, reduced to something you could start tonight.
- Fix the bedtime, not the sleep. Work backwards from when you wake, add seven hours, and put that in your calendar. This is the single highest-value item on the page.
- Walk most days. Not a programme. The evidence is about frequency, not intensity, and the biggest gains are at the least active end.
- Name the chronic stressor, if there is one. If something has been running for months with no end in sight, that is the health variable. Treat it as one.
- Eat enough, with enough protein. Particularly if you have been busy, or cutting, or both.
- Take vitamin D daily through winter rather than occasionally in large amounts. Daily beat bolus in every analysis.
- If you are going to take vitamin C, take it before you need it. Starting at the first symptom is the one approach the evidence does not support.
- Pick one thing to reduce. Alcohol most likely. One, properly, for a month beats five for a week.
Frequently asked questions
How many colds a year is normal?
Two to four a year is the usual range for an adult, and more if you have young children or work with the public. So three colds in a winter is not a sign that anything is wrong. What is worth looking at is a change from your own baseline, infections that keep dragging on for weeks, or repeated infections that need antibiotics.
Does being cold or wet actually make you sick?
Not directly. You catch colds from viruses, not from temperature. What is true is that winter puts more people indoors together with less ventilation, and cold dry air changes how well the lining of your nose clears virus particles. So the season matters, but going out with wet hair is not the reason.
Does vitamin C stop you getting a cold?
Mostly it does not stop you catching one, and that is the wrong job to give it. What it does do, when taken daily beforehand at 1g or more, is make colds around 15 per cent less severe and about 8 per cent shorter. The exception is people under extreme sustained physical stress, where regular vitamin C roughly halved the rate of colds. The thing worth changing is timing: started only once symptoms appear, it has not shown a consistent effect.
Is it my immune system or just bad luck?
Usually neither, and that is the useful answer. Most people who feel like they are always sick are carrying one or two specific things that are known to raise susceptibility: consistently short sleep, a long-running stressful situation, very little physical activity, or a winter with almost no sun exposure. Those are measurable and changeable, which is a better position to be in than luck.
Should I take immune supplements all year or only in winter?
It depends on the nutrient. Vitamin D is the one with a clear seasonal argument in New Zealand, because your skin makes very little of it from winter sun at our latitude. The others are daily nutritional intakes rather than seasonal interventions. Worth knowing that in the vitamin D research, steady daily dosing outperformed occasional large doses, so consistency matters more than timing.
When should I see a doctor about getting sick so often?
See a doctor if you are having repeated infections that need antibiotics, infections that keep coming back in the same place, unusual infections, infections that take far longer to clear than they used to, or if frequent illness comes with unexplained weight loss, night sweats, or persistent swollen glands. Frequent minor colds are rarely a sign of immune deficiency. That pattern is.