How the ‘superflu’ surging can kill you and the 6 easy-to-miss signs it’s turning deadly

December 12, 2025

FLU is tearing through NHS hospitals ahead of Christmas, and Health Secretary Wes Streeting has sounded the alarm over fatalities.

There are thousands of deaths from flu every year in the UK, and experts previously told the Sun they feared “tens of thousands” could die this winter. Some people have up to seven times more at risk than others – and there are 6 signs you need to look out for that mean you should call 999.

For many, a flu infection is part and parcel with winter – you feel rotten for a week or so, but you’re up and running shortly afterwards.

But for some, this seemingly common infection can have dangerous – and sometimes deadly – consequences, as it can trigger severe complications, from pneumonia, to heart attacks or sepsis.

An estimated 7,757 people died of influenza-related causes in England during the 2024/25 flu season, according to the UK Health Security Agency – double the number of deaths seen over the previous season.

Now, as a “tidal wave” of mutant superflu tears through the UK, Health Secretary Wes Streeting has sounded the alarm over fatalities – especially as hospitals brace for a five-day strike by resident doctors next week.

He warned the NHS may buckle and “people could die” if strikes go ahead while the health service faces a “worst-case scenario”.

Last week, an average of 2,660 patients were in hospital with the bug each day – the highest ever for this time of year and 55 per cent up on last week.

The infections are being driven by strain of flu, dubbed ‘subclade K’ or H3N2, which Sun columnist and GP Dr Zoe Williams warned is “more severe” than normal flu and “more infectious”.

The strain mutated several times over the summer and is different to ones we’re usually exposed to each flu season, meaning people have built up less immunity against it.

According to Prof Ed Hutchinson, from the University of Glasgow Centre for Virus Research, explained there tend to be three types of influenza virus causing winter illness – H3N2, H1N1 and influenza B virus.

“They all continually acquire mutations which mean that our immunity against them goes out of date,” he said.

“At the end of this year’s flu season in the southern hemisphere (i.e. our summer) the H3N2 strain of the virus suddenly acquired seven new mutations.

“This is a lot more than normal, and it meant that when the flu season took off in the northern hemisphere we had less protection than normal against the H3N2 component of seasonal flu.”

Dr Andrew Conway Morris, charity medical director for Sepsis Research FEAT, told Sun Health: “This year the dominant strain is called H3N2.

“H3 strains tend to produce more severe disease in older people than the H1 strain that caused the 2009 ‘swine flu’ pandemic.

“The reasons for this are not fully understood, but an increase in severe illness in older people is likely to increase the rates of hospitalisation, putting pressure on the NHS.

WHY CAN FLU BE DEADLY?

While most people who have the flu will recover at home, certain people will be at higher risk of becoming seriously ill from the infection and needing to go to hospital.

At-risk groups include young children, people over the age of 65, pregnant women and those with underlying health conditions.

Karen Spillett, head of health advice at Asthma + Lung UK told Sun Health: “Even if you do not have an underlying health condition it can make you feel very ill.

“However, if you have an underlying health condition you are more at risk of experiencing serious complications from it, which is why it’s crucial to get a free flu vaccine if you’re eligible.”

Here, we explain who is most vulnerable to the flu and what complications it can cause.

1. It can cause pneumonia

Conditions that can up someone’s risk of severe illness or complications from the flu include lung conditions like asthma and chronic obstructive pulmonary disease (COPD).

People with these conditions are “seven times more likely to die from flu compared to healthy individuals”, according to Dr Andy Whittamore, clinical lead at Asthma + Lung UK.

Karen said: “Flu can be bad for anyone, but if you have a lung condition such as asthma or COPD, it can be much more serious.

“It can trigger severe flare-ups of someone’s existing lung condition, increasing inflammation in the airways, making symptoms like wheezing and breathlessness worse.

“This increased inflammation can bring on a life-threatening asthma attack in people with asthma or serious flare-up in those with COPD.”

An asthma attack can show up as a cough, wheezing, breathlessness, and chest tightness.

A COPD flare-up can mean usual symptoms of the lung condition – caused by damage and inflammation to the lungs – becomes worse.

This can cause breathlessness, coughing and phlegm.

A flu infection can also lead to “serious conditions, such as bronchitis and pneumonia, which can be fatal”, Karen added.

She explained: “The flu virus attacks the respiratory system, and for those with pre-existing lung issues, this can lead to a more severe illness, which is harder for the body to fight off.”

Signs of pneumonia

According to the NHS, symptoms of pneumonia can start suddenly or gradually over a few days.

They include:

A cough – you may cough up yellow or green mucus (phlegm)
Shortness of breath
A high temperature
Chest pain
An aching body
Feeling very tired
Loss of appetite
Making wheezing noises when you breathe – babies may also make grunting noises
Feeling confused – this is common in older people

Unlike colds or flu, pneumonia often causes difficulty breathing and a sharp or stabbing pain in the chest that worsens with deep breaths or coughing.

If these symptoms appear, it’s important to seek medical advice promptly.

In an emergency, always call 999.

Source: NHS

2. It can cause a heart attack

Flu can be particularly dangerous to people with cardiovascular disease.

According to the British Heart Foundation (BHF), the virus can weaken the respiratory system, which is important for the heart to function properly.

This can directly harm the heart muscle, the charity said.

Data from the winter of 2019/20 shows there were over 6,300 excess deaths from cardiovascular disease in the UK, with flu likely to be a contributing factor.

Ruth Goss, senior cardiac nurse at the BHF said: “Anyone with a history of heart and circulatory illnesses is at increased risk of severe illness if they get the flu.

“If you have had a heart attack, the flu puts you at a greater risk of having another one, and it can exacerbate conditions like heart failure.”

Symptoms of a heart attack include a feeling of pressure, heaviness, tightness or squeezing across your chest, as well as pain in the arms, jaw, neck and back.

Heart failure occurs where the heart can’t pump blood effectively around the body.

Symptoms like breathlessness, fatigue, and swollen legs tend to come on gradually.

3. It can cause sepsis

In rare cases, flu can cause sepsis. This is when the body overreacts to an infection and attacks its own tissues and organs.

It can be life-threatening.

Dr Conway Morris said: “Influenza can cause sepsis in two different ways.

“The virus itself can cause severe lung damage and a ‘viral pneumonia’ that can cause sepsis.

“Influenza also increases the risk of getting other lung infections, both bacterial and fungal.

“Flu and patients who get these so-called ‘co-infections’ are even more likely to develop sepsis and need management in an intensive care unit.

“The vast majority of people who catch flu will not get sepsis, and simply need to rest up, take in plenty of fluids and use simple medicines such as paracetamol,” he stressed.

Warning signs of sepsis to look out for include:

Confusion or excessive drowsiness
Uncontrolled shivering
Increased respiratory rate
Very cold hands and feet, especially if accompanied by a rash or mottled skin
High temperature which doesn’t respond to paracetamol or low temperature (less than 36C)
Not passing urine for more than six to 12 hours.

“Overall, sepsis makes people feel more unwell than they ever have before, so if you are in that state or a loved one appears more unwell than you have ever seen them then you should seek urgent medical attention,” Dr Conway Morris added.

4. It can worsen diabetes

If you have diabetes and get the flu, you are more at risk of being unwell for longer and developing more serious complications like pneumonia, Diabetes UK warned.

Flu can also make your diabetes harder to manage and cause your blood sugar levels to rise.

It can also up your risk of developing a life-threatening condition called diabetic ketoacidosis (DKA).

This is caused by a severe lack of insulin. Without insulin your body can’t move sugar into the cells for energy so instead breaks down fat releasing harmful chemicals called ketones which build up and make your blood acidic.

A large build-up of ketones can lead to you becoming seriously ill very quickly.

Symptoms of diabetic ketoacidosis can include:

Being very thirsty 
Needing to wee more often 
Feeling tired and sleepy 
Confusion 
Blurred vision 
Stomach pain 
Feeling or being sick 
Sweet or fruity-smelling breath, like nail polish remover or pear drop sweets 
Passing out

If you spot any of these symptoms it is a sign that you need to get some medical help quickly. 

WHEN TO CALL 999

If you get struck down by the flu, the best thing you can do is stay home and rest, sleep and drink plenty of fluids.

You should start feeling better after a week or so.

But watch out for six red flag symptoms in yourself or your child.

Phone 999 or go to A&E if:

You have severe difficulty breathing – for example, you struggle to speak without pausing, gasping or choking
You have severe chest pain in the middle of your chest that isn’t going away – it may feel like pressure, tightness or squeezing
You have pain that spreads to your arms, back, neck and jaw
You feel more drowsy than usual or find it more difficult to wake up
Your lips or skin are turning very pale, blue or grey – on brown or black skin this is easier to see on the palms of your hands
You start coughing up blood

HOW TO PROTECT YOURSELF IF YOU’RE AT RISK

Hoping to dodge the superflu tearing through the UK?

One way to protect yourself is very simple – make sure to wash your hands regularly with warm water and soap.

And if you haven’t already, getting your flu jab can help protect you from getting sick and developing severe illness.

Dr Conway Morris said: “This year’s seasonal flu vaccine does contain an H3N2 strain but the virus has mutated since the vaccine was put together.

“Encouragingly, however, early data from UKHSA suggests that being vaccinated does reduce the risk of hospitalisation amongst both adults and children to a similar degree to previous years vaccines.

“It is therefore highly advisable that eligible people such as those over 65 or those with underlying health conditions including asthma and diabetes get vaccinated.”

At-risk groups can get their vaccine on the NHS for free, but you can also buy it privately at chemists and supermarket pharmacies.

Prices start at around £15 – read our piece on where you can find the cheapest jab.

You may want to consider wearing a face mask too.

Dr Whittamore said: “While the government does not specifically recommend that the general population wear a mask to protect themselves from viruses, Asthma + Lung UK is urging people with lung conditions to consider wearing a face mask in crowded or indoor spaces to reduce their chance of catching flu.

“Masks can help reduce the chance of inhaling viruses, but staying up to date with vaccinations, practicing good hand hygiene, and avoiding contact with sick people are the main ways to stay safe.”