Constantly catching colds? The most common causes of frequent infections – and what you can do about them

*Note: The header image is AI-generated

The pack of tissues in your jacket pocket is almost empty again, and the last cold feels like it just ended. Someone is coughing at the office, everyone is sneezing on the train, and your throat is scratchy again. At some point, you wonder if something is wrong with your immune system. If you constantly need tissues, it can be hard to tell what's normal. Frequent colds in adults usually aren't a cause for concern, as two to four infections a year are considered normal. What matters is whether they last a long time or are severe.

In this article, you'll learn how many colds are still considered normal, what studies show about sleep and stress, what nutrient deficiencies might be to blame, which blood tests can help your doctor, and how to take a closer look at your daily life.

Table of Contents

Frequent colds: How many colds a year are still normal?

Two to four colds a year are considered normal in adults. Infectious disease specialists also see no cause for concern with three or four infections a year. Children, by the way, are significantly higher, with six to ten colds a year.

But the number alone tells you surprisingly little. What's more relevant to doctors is how your infections develop: Do they last unusually long? Do they become unusually severe? Do you need antibiotics again and again? A concrete indication that further investigation is needed is four or more infections a year requiring antibiotics.

It's completely understandable if the gray area still doesn't fully reassure you. Many working adults fall into this category, and for them, it's worth looking at the two factors that have been best studied in research.

Stress and sleep deprivation: What studies really show

Lack of sleep and ongoing stress increase the risk of actually getting sick after exposure to a cold virus. This isn't just a hunch: researchers have measured it, and with a fairly direct experimental setup.

In short: In a viral challenge study, healthy volunteers in quarantine are deliberately given a cold virus in their nose via nasal drops. Researchers then observe over several days who develops symptoms and in whom the virus can be detected. This allows them to compare who gets sick after the same exposure to the pathogen.

The best-known study on this was conducted by Sheldon Cohen's research group. 153 healthy women and men between 21 and 55 years old kept a log for two weeks of how long and how well they slept. Then they received rhinoviruses via nasal drops and were observed for five days. Less than seven hours of sleep increased the risk of illness about threefold compared to eight hours or more (1).

The catch with this first study: The participants recorded their sleep themselves, and people are known not to always estimate their sleep duration reliably. A follow-up study with 164 adults between 18 and 55 years old therefore measured it objectively, with a motion sensor on the wrist over seven days.

The result remained: Shorter sleep was associated with a higher susceptibility to colds (2). And stress? The same research group also investigated this under quarantine conditions. People under prolonged stress were more likely to get sick after exposure to the virus. This connection has been shown with various cold viruses and may be explained by glucocorticoid resistance: Immune cells respond less effectively to the body's own cortisol under chronic stress, which normally keeps inflammation in check.

To be fair: These are studies on healthy volunteers with artificial exposure to the virus. They show a clear connection, but the exact mechanism is not fully understood.

There is currently no study that specifically examines working adults with frequent colds and the combination of stress, sleep deprivation, and nutrient gaps. Nevertheless, the message for your daily life is clear: Sleep is one of the few factors you can influence yourself.

What nutrient deficiency might really be to blame

A deficiency in vitamin D, zinc, or iron can weaken your defenses. But according to current knowledge, a supplement will only help if a deficiency actually exists.

Vitamin D is the best studied. In observational studies, people with low vitamin D levels have respiratory infections more often. Whether taking it protects against them is another question. A widely cited meta-analysis evaluated data from 25 double-blind studies: In people with very low baseline levels below 25 nmol/l, vitamin D reduced the risk of infection by around 70 percent; no such effect was seen in people with sufficient levels (3).

However, a later re-evaluation indicated that the result depended heavily on a single study involving schoolchildren in Mongolia. Newer meta-analyses find no statistically significant protection for the general population.

Zinc: Its role in the immune system is well established. Phagocytes and lymphocytes need it to function, and a deficiency can increase susceptibility to respiratory infections. There is early evidence from small studies that zinc supplementation can reduce the number of infections in older people. Iron is the most sensitive candidate. A deficiency is associated with increased susceptibility to infections, but too much iron without a deficiency can actually play into the hands of pathogens, because the body deliberately keeps iron away from them during infections.

The vitamin C effervescent tablet? For the general population, review articles so far show no reliable effect of vitamin C supplementation on the number of colds. So if you only reach for a tablet when your throat starts to scratch, you're addressing the wrong thing.

If you want to know more about what role different vitamins play in the immune system, this article might be interesting for you: Vitamins for the immune system and their roles.

Which blood tests really help your doctor

The three most revealing values for frequent infections are 25-OH-vitamin D, ferritin, and, depending on your diet, zinc. None of them are part of the statutory health check-up from age 35, so you need to ask for them specifically. It's quite possible that you'll have to pay for individual tests yourself if there's no concrete medical reason.

When the results are in, take your time to look at them. Especially with vitamin D, many people stumble over the units.

The unit trap: Studies often report vitamin D in nmol/l, while many results are in ng/ml. Divide the nmol/l value by 2.5 to get ng/ml. The threshold of 25 nmol/l from the meta-analysis therefore corresponds to only around 10 ng/ml, a value significantly below the usual laboratory thresholds.

Some situations should be discussed with your family doctor promptly:

When talking to your doctor, it helps to be specific: how many infections in the last twelve months, how long each one lasted, how often antibiotics were necessary, what your diet is like, and what medications or supplements you take. With this information, "I'm constantly sick" becomes a question your doctor can work with. And the sentence "That's completely normal" then gets an explanation.

Your daily life, environment, and plan for the next few weeks

Frequent colds are often simply due to more exposure to viruses. Experience shows that a child in daycare or primary school brings home every pathogen that is currently going around there. An open-plan office, a crowded train on the way to work, or a new job with new colleagues cause the same thing. And between October and March, cold viruses are already in high season.

Before you look for a deficiency or immune weakness, it's worth taking an honest inventory: How long have you been getting sick so often, and has anything changed in that time, such as your job, living situation, or a child starting daycare? How many hours do you really sleep on a normal workday? When was the last time you had a period without pressure from appointments? Do you eat mostly plant-based or rather irregularly? And do your infections come in clusters in winter or are they spread throughout the year?

The answers are not a diagnosis, but they sort out your most likely causes and should be discussed with your doctor.

From this, you can create a plan that will be most effective in this order:

If you want to delve deeper into how you can support your defenses in your daily life, this article might be interesting for you: Strengthen the immune system, quickly and long-term.

A supplement makes sense then if a deficiency is likely, such as with a vegan or very irregular diet. It does not replace a medical assessment. When using a multivitamin as a base, it's worth looking at three things: quantities around the daily requirement instead of megadoses, well-absorbed compounds, and as few additives as possible. Vitamin C, vitamin D, zinc, and selenium contribute to the normal function of the immune system. If a supplement contains iron, it makes sense to check your ferritin level beforehand.

If a zinc deficiency is likely, such as with a plant-based diet, CLAV Zinc Bisglycinate 25 mg is an option: 25 mg of zinc as bisglycinate per tablet (250% of the reference value), 180 tablets per container, from €16.90. Zinc contributes to the normal function of the immune system. At 25 mg, this is a high dose, which you should best coordinate with your doctor or pharmacy. Click here for the product: View Zinc Bisglycinate 25 mg.

You can find an overview of other products related to defenses in the CLAV Immune System category.

Important for all supplements: Do not exceed the recommended daily amount. They are not intended for children or pregnant women. Zinc and iron inhibit each other's absorption if taken at the same time. Both can also reduce the absorption of certain antibiotics (such as tetracyclines or quinolones) and thyroid hormones. If you take medication, clarify the time interval in the pharmacy or practice.

The next step instead of the next effervescent tablet

Frequent colds are rarely caused by a single factor. Usually, it's two or three factors together: too little sleep, a lot of pressure, an environment full of viruses, and perhaps a nutrient gap that no one has measured yet.

Your next step, therefore, is not a new supplement, but an appointment. Take your answers to the questions about sleep, stress, and daily life with you and discuss 25-OH-vitamin D, ferritin, and, depending on your diet, zinc and B12. In parallel, you can start with the sleep log today. Whether a supplement makes sense afterwards, you will decide then based on real values instead of guesswork.

FAQ: What you want to know about frequent colds

Can you train the immune system through frequent colds?

Only to a very limited extent. Although your body remembers the individual pathogen, there are many different cold viruses. A cold you have recovered from therefore hardly protects you from the next one, and many infections are no sign of a particularly well-trained immune system.

How long does it take for vitamin D or ferritin values to be available?

Usually a few days to about a week. Ferritin and the blood count are part of the routine of almost every laboratory; vitamin D and zinc are sometimes sent to specialized laboratories and take a little longer. It's best to ask directly at the time of the blood draw when you can call back.

Does the normal frequency of colds change with age?

Rarely in number, rather in course. Older adults do not necessarily get sick more often, but often more severely, because immunological memory declines with age. This is why long or severe courses in old age deserve more attention than the pure number.

Should you have blood tests repeated after some time?

Yes, if a value was unusual or if you have changed something. Vitamin D fluctuates with the seasons, and ferritin can be distorted immediately after an infection. A check after a few months shows whether something has really changed.

Do stress, sleep deprivation, and nutrient gaps reinforce each other?

This is usually obvious, but has hardly been studied. Each factor on its own is associated with increased susceptibility, but studies on their combination are currently lacking. In practice, this means: Those who take action in two areas at once, such as sleep and checked levels, have the best chance.

Is the vitamin D test covered by health insurance?

Not always. If there is a specific medical reason, health insurance usually covers the test, but you often have to pay for it yourself for purely preventive purposes. Speak openly in the practice about why you would like to know the level, and this can be clarified beforehand.

Sources

(1) Cohen S, Doyle WJ, Alper CM, Janicki-Deverts D, Turner RB (2009): Sleep habits and susceptibility to the common cold. Archives of Internal Medicine 169(1): 62–67. PubMed, PMID: 19139325.

(2) Prather AA, Janicki-Deverts D, Hall MH, Cohen S (2015): Behaviorally assessed sleep and susceptibility to the common cold. Sleep 38(9): 1353–1359. PubMed, PMID: 26118561.

(3) Martineau AR, Jolliffe DA, Hooper RL et al. (2017): Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 356: i6583. PubMed, PMID: 28202713.