Cold, flu or Covid: what's the difference?

Daniel Platteau

In short: the common cold is caused by more than 200 different viruses, mostly rhinoviruses: it is unpleasant but harmless, and its symptoms come mainly from our own immune reaction. Flu and Covid are caused by more aggressive viruses, which do more damage to the airways, trigger stronger inflammation and can leave lasting effects: a six-fold higher risk of heart attack in the week after flu, and long Covid in about 6 out of 100 people infected. Cold weather does not make you ill, but it weakens the nose's defences and helps viruses spread. What really protects you: vaccination (flu and Covid, for people at risk), hand washing, enough sleep and staying at home when you are ill. Vitamins C and D do not prevent infections.

Updated 4 November 2026.

The days are getting shorter, the leaves are falling, and the tissues are coming out. Every autumn, the same "gifts" come back: the common cold, flu, Covid, not to mention RSV (respiratory syncytial virus), which mainly affects babies and older people. They are often confused. Yet they are very different viruses, which do not attack us in the same way and do not leave the same marks. Here is how to tell them apart, how they infect us, and what really works to protect yourself.

Cold, flu or Covid: how to tell them apart?

The symptoms overlap, and only a test can settle it. But a few signs point the way.

An adult catches on average two to four colds a year, a young child six to eight. Flu and Covid are less frequent, but they are the ones that fill hospitals in winter. In Belgium, the last flu epidemic lasted 13 weeks, from early December 2025 to the end of February 2026.

Three viruses, three families

A virus is not a self-sufficient living thing. It is a small packet of genetic material (DNA or RNA, the "recipe" for making new viruses), wrapped in a protein shell called a capsid. It can only multiply by entering a cell and hijacking its machinery.

Rhinoviruses, responsible for about half of all colds, are naked viruses: nothing surrounds their capsid. More than 160 different types are known. That is why nobody has ever managed to make a vaccine against the common cold: it would have to cover dozens of them.

Flu viruses and SARS-CoV-2 are enveloped viruses: their capsid is surrounded by a thin fatty membrane, stolen from the cell they leave. This envelope helps them enter our cells and evade our defences, but it is fragile. Soap and alcohol-based hand rubs destroy it quickly.

Other viruses also cause colds: "ordinary" coronaviruses (about 7 to 18% of adult colds), RSV, adenoviruses, metapneumovirus. And in roughly one cold in four, no known virus is found.

How they infect us

A key and a lock

To enter a cell, a virus must latch onto a molecule on its surface, a bit like a key in a lock. Each virus has its own key.

• The rhinovirus mainly attaches to a protein called ICAM-1, found on the cells lining the nose. It multiplies better between 33 and 35 °C, the temperature inside the nose, than at 37 °C. So it stays quietly in the upper airways.
• The flu virus carries two proteins on its surface that give it its code name: haemagglutinin (H), which attaches to sugars (sialic acid) on the cells of the nose, throat and lungs, and neuraminidase (N), which helps it break free to infect the next cell. Hence names such as H1N1 or H3N2.
• SARS-CoV-2 uses its "spike" protein (the spikes that form the coronavirus's crown) to attach to the ACE2 receptor. This receptor is found in the nose, but also in the lungs, blood vessels, heart, kidneys and gut. That is one of the reasons why Covid can affect the whole body, even though some of this damage comes from inflammation rather than from the virus itself.

Viruses that never stop changing

All three have an RNA genome. To copy themselves, they use an enzyme which, unlike those in our cells, corrects few or none of its mistakes (coronaviruses have a rudimentary proofreading system, rhinoviruses and flu viruses none). Their copies therefore contain small errors: mutations. Most are useless or harmful to the virus, but some allow it to escape the antibodies made during a previous infection.

As a result, our immune system "remembers" a virus, but not necessarily its cousin next year. That is why you can catch several colds a year, and why flu and Covid vaccines have to be updated every year. In August 2025, a new variant of the H3N2 virus, called "subclade K", appeared and spread very quickly around the world. It brought forward the start of the flu season in several countries, and the World Health Organization (WHO) changed all three strains in the 2026-2027 vaccine. As for Covid, the XFG variant is dominant in Europe this autumn, with no sign of increased severity according to the European Centre for Disease Prevention and Control (ECDC).

Why a cold is (mostly) our own fault

Here is the paradox of the common cold: the rhinovirus does very little damage to our nose. It infects only a small fraction of cells and hardly destroys them at all. The symptoms come mainly from our own defences.

As soon as a cell detects the virus, it releases cytokines, messenger proteins that raise the alarm and attract white blood cells. To get these reinforcements there quickly, the body sets off a cascade that produces bradykinin. This small molecule widens blood vessels and makes them leakier. In 1988, researchers at Johns Hopkins University sprayed bradykinin into the noses of healthy volunteers: they got a runny nose, a blocked nose and a sore throat, without any virus.

• A blocked nose comes mainly from swollen nasal tissue, far more than from mucus. That is why you can blow your nose endlessly without clearing it.
• A runny nose comes from the mucus glands, stimulated by inflammation.
• A sore throat comes from local inflammation and from cytokine-laden mucus running down the throat.
• Tiredness, the urge to stay under the duvet, is a behaviour the body wants: cytokines act on the brain to push us to save energy.
• Fever is a thermostat deliberately turned up by the brain, which boosts our defences and hampers the multiplication of viruses.

Why so much effort for an almost harmless virus? Because the immune system does not know in advance what it is dealing with. Reacting fast and hard to anything that looks like a virus is a survival strategy, even if it ruins our week.

Flu and Covid are different. These viruses do more damage to the cells they infect, travel more easily down to the lungs and trigger a much stronger inflammatory reaction throughout the body. That explains the aching muscles, the high fever and, in vulnerable people, pneumonia.

Science communicator Bruce Benamran explains this mechanism very well in a recent video on his French-language channel e-penser, devoted to the common cold:

Why in autumn and winter?

"Catching a chill" is a misleading phrase: without a virus, you can be as cold as you like without falling ill. But cold weather really does help viruses, in several ways.

• A cold nose defends itself less well. In 2015, a team at Yale University showed, in mouse cells, that the antiviral response against rhinovirus is weaker at 33 °C than at 37 °C. In 2022, a team in Boston discovered that nasal cells release billions of tiny vesicles that act as decoys for viruses. When volunteers breathe air at 4 °C for a quarter of an hour, the inside of their nose cools by about 5 °C, and at that temperature the cells produce about 42% fewer vesicles (laboratory study, on human tissue).
• Cold, dry air helps flu travel. In guinea pigs, the flu virus spreads much better at 5 °C than at 20 °C, and better in dry air (20 to 35% humidity) than in humid air. Droplets evaporate quickly and leave tiny particles that float for a long time.
• We live shut in and crowded together. Closed windows, packed public transport, schools back in session: the opportunities for contagion multiply.

These results come mainly from laboratory and animal studies. They explain the cold season well, but do not allow us to put a figure on the effect of a scarf over your nose.

And in the long run?

A cold usually heals without after-effects, even if the cough can linger for several weeks. It can also open the door to a bacterial infection (ear infection, sinusitis) or trigger an asthma attack. For flu and Covid, the consequences can last longer.

The heart. A Canadian study published in 2018 in the New England Journal of Medicine showed that the risk of being hospitalised for a heart attack is six times higher in the seven days after a laboratory-confirmed flu infection. After the first week, the risk returns to normal. Inflammation can destabilise cholesterol plaques already present in the arteries. Other respiratory viruses also increase this risk, but less.

Long Covid. According to the WHO, about 6 out of 100 people who get Covid develop long Covid: symptoms that appear within three months of the infection, last at least two months and cannot be explained otherwise, such as fatigue, breathlessness, pain or sleep problems. Most people improve within 4 to 9 months, but about 15% still have symptoms after a year. The risk seems lower today than at the start of the pandemic, and vaccination reduces it.

"Long flu". An American study from 2023 followed more than 90,000 people hospitalised for Covid or flu for 18 months. Both groups had more health problems and deaths in the months after the infection than during the acute phase. Covid affected more organs (heart, brain, kidneys), flu mainly the lungs. An important limitation: these were hospitalised patients, mostly older male veterans, not the general population.

How to protect yourself?

Many products promise to "boost your immunity". Here is what the studies actually show.

A few useful details.

• Soap or gel? Both destroy the envelope of the flu virus and SARS-CoV-2. On human skin, in the laboratory, SARS-CoV-2 remained infectious for about 9 hours and the flu virus for about 2 hours, but an 80% alcohol solution inactivated both in 15 seconds. The rhinovirus, which has no envelope, is more resistant to chemicals and survives longer on surfaces and hands. Washing with soap, which physically removes viruses, remains the basis.
• Masks have not proven themselves as a general measure. They remain sensible for a sick person who has to go out, or to protect a vulnerable person.
• Airing rooms for a few minutes several times a day lowers the concentration of virus in the air. It is logical and risk-free, even if trials are lacking to put a figure on the effect.
• Staying at home when you are ill is probably the most useful thing you can do for others.
• Antibiotics are useless against a virus. They only help in the case of a secondary bacterial infection, on medical advice.

For people at risk, antiviral drugs exist against flu and Covid. They must be started early: ideally within 48 hours for flu, within the first five days for Covid. If you have symptoms, call your doctor without waiting for things to get worse.

True or false?

"I caught a chill." False. Cold alone does not give you a cold: you need a virus. But the cold weakens the nose's defences and helps viruses spread.

"Flu is just a bad cold." False. They are different viruses. According to the WHO, flu causes 3 to 5 million cases of severe illness and 290,000 to 650,000 respiratory deaths worldwide every year.

"An antibiotic will help me get better faster." False. Antibiotics do not act on viruses. They do, however, encourage resistance and disrupt the gut flora.

"Covid has become just a cold." Partly true. Thanks to immunity from vaccines and infections, Covid is much less severe than in 2020. But it remains dangerous for older and vulnerable people, and the risk of long Covid still exists.

"Blowing your nose often unblocks it." False. The nose is blocked mainly because its tissues are swollen, not because it is full of mucus.

"Vitamin C prevents colds." False for the general population. It only reduced the risk in people under extreme physical stress (marathon runners, soldiers exercising in severe cold).

A bit of history

1889-1890, the "Russian flu". This pandemic killed about a million people worldwide. It was long attributed to flu. In 2005, a team at the Rega Institute of KU Leuven compared the genome of a common-cold coronavirus, OC43, with that of a cattle coronavirus. Their calculation placed the common ancestor around 1890. The hypothesis: a coronavirus jumped from cows to humans, caused a deadly pandemic, then became a simple cold virus. The symptoms described at the time, such as loss of taste and smell, are reminiscent of Covid. This appealing hypothesis remains debated: direct proof is lacking.

1918-1920, the "Spanish" flu. An H1N1 virus infected about a third of humanity and killed at least 50 million people, mostly young adults. It reached occupied Belgium in June 1918, probably with German soldiers; the second wave, in October, was the deadliest. The exact number of Belgian victims is not known, probably tens of thousands.

1933, the virus is finally identified. In London, three researchers dripped the throat washings of a colleague with flu into the noses of ferrets. The ferrets fell ill: flu is indeed caused by a virus, and not by a bacterium as was often thought.

1946-1989, hunting the common cold. In the United Kingdom, the Common Cold Unit, set up in a former military hospital near Salisbury, welcomed thousands of volunteers who were given colds on purpose. It never found a cure, but it identified many viruses. In 1965, David Tyrrell's team described an unknown virus there, named B814.

2019, Covid. A new coronavirus, SARS-CoV-2, caused the pandemic we all know. More than 7 million deaths have been reported to the WHO, and estimates based on excess mortality are much higher.

Key takeaways

The common cold is a mild infection whose symptoms we largely produce ourselves. Flu and Covid are in another category: they can kill vulnerable people and leave after-effects, on the heart and elsewhere. No dietary supplement really protects us from them. The effective measures are simple: vaccination for those for whom it is recommended, soap, sleep, fresh air, and staying at home when you are ill, so as not to pass on to your neighbour the gift you received.

Going further

Why do these viruses mutate so much?

Our cells copy their DNA with an enzyme that proofreads its work: it makes about one error per one to ten billion "letters". Rhinoviruses and flu viruses use an enzyme without proofreading, which makes a mistake about once every thousand to ten thousand letters. For a rhinovirus, whose genome has about 7,200 letters, each copy can contain a few errors. Coronaviruses are a partial exception: their genome, four times longer, has a rudimentary correction enzyme, which slows their evolution somewhat.

Drift and shift: how flu pandemics arise

Small successive mutations are called antigenic drift: they explain the epidemics every winter. More rarely, two different flu viruses (for example a human virus and a bird virus) infect the same cell and swap pieces of genome. This is antigenic shift: a completely new virus appears, to which nobody is immune. This mechanism lies behind several flu pandemics, including the 2009 one, and it is why the WHO keeps a close eye on avian flu.

Why does cold weaken the nose's defences?

In the Yale study, cells infected at 37 °C produced far more interferons, antiviral alarm signals, than at 33 °C. In mouse cells genetically deprived of these signals, the virus multiplied more, including at 37 °C. So it is mainly our defence, and not the virus itself, that depends on temperature.

Test yourself

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