When a peer-reviewed article in the British Medical Journal suggested last summer that Covid may have been quietly weakening our immune systems, it passed largely unnoticed.

Its author, Canadian science writer Nick Tsergas, was careful not to overstate the case – the science, he stressed, was still evolving. But the question he raised was unsettling – whether the virus might have left lasting immune changes, even in people who believed they had recovered completely.

For the past few years, rising infection rates have been explained by the so-called 'immunity debt' theory – the idea that the pandemic lockdowns and social distancing suppressed the circulation of everyday viruses, leaving people more vulnerable once normal life resumed.

As restrictions lifted, bugs returned, and many assumed we were simply paying the price all at once.

But more than five years on, that explanation is beginning to look incomplete, Mr Tsergas argued. Outbreaks have not faded away. In some cases, they appear to have intensified – prompting scientists to ask whether something else may be going on.

In the piece, Mr Tsergas quoted Dr Samira Jeimy, a clinical immunologist at the University of Western Ontario, who said she had seen an alarming rise in mycoplasma pneumoniae – a milder but still serious form of pneumonia, sometimes dubbed 'walking pneumonia', which typically affects younger people.

'I can count on my hands the number of times I'd ever seen mycoplasma pneumoniae before 2023,' she said. 'All of a sudden, I feel like everybody has it.'

As the UK continues to be battered by a severe winter flu outbreak – with hospitalisations up by about 10 per cent in the past week – the article has begun circulating again on social media. In one Insta­gram video, a user posting under the name Paco­OnPause says: 'I keep seeing people say, 'This is the sickest I've ever been.'

'You're going to hate this, but are you sickest now because you keep getting Covid?'

In another clip posted last month, a young woman rejects claims that Covid is 'just like flu', saying she has 'developed a new chronic illness every time she gets infected'. A third asks: 'Why have I had a cold every two months since having Covid?'

UK doctors say they are seeing similar patterns emerge.

'I have definitely seen this,' said Kent-based GP Dr Stephanie De Giorgio. 'Young people are getting more serious complications from viral infections, such as pneumonia and tonsillitis leading to abscesses in the throat.

'We're seeing more cases of glandular fever and referring more young patients for secondary-care [specialist] investigations than we needed to before. We can't keep pretending a pandemic didn't happen, or that it hasn't had a long-term impact on our health.'

Last week The Mail on Sunday's columnist and GP Dr Ellie Cannon also raised concerns, writing: 'Some of the stories I've heard are startling – fit, healthy people in their 30s and 40s developing pneumonia, sepsis and shingles, conditions usually associated with the frail and elderly.

'Many patients feel they're battling one cough or cold after another. It makes you wonder if something else might be going on.' We received a flood of emails from readers who said they shared those suspicions.

One woman, aged 51, wrote that a minor infection last year worsened to the extent she needed hospital treatment. 'Before that, I considered myself healthy. I was very rarely sick,' she said. 'But since then I've lost count of the number of times I've been prescribed antibiotics.'

Another reader, in her 60s, said: 'I caught a strep throat infection that turned into sepsis. I was in intensive care for ten days. Since then, I've been ill constantly and seem to catch every infection going.'

So what is going on? Could repeated Covid infections really have left some immune systems less able to cope?

There is evidence that, since the pandemic, we have been seeking medical advice for illness far more often.

Last year there were more than seven million calls to the NHS non-emergency helpline 111 – an average of about 660,000 a month – compared with a pre-pandemic average of 155,000.

UK health surveillance data shows there has indeed been a marked rise in mycoplasma pneumoniae, the bacterial infection that causes so-called 'walking pneumonia'.

Cases surged during the winter of 2023, particularly among children and young adults, and levels have remained higher than expected since – prompting warnings from public health officials that its spread has not yet returned to pre-pandemic norms.

A series of recent studies also suggests we may, overall, be getting ill more often.

A 2025 paper published in The Lancet tracked more than 830,000 US veterans and found that even patients who were not hospitalised for Covid had higher rates of bacterial, viral and fungal infections in the year that followed.

A growing number of scientists believe this may be because Covid can subtly alter immune function.

Last month, a large study published in the International Journal of Infectious Diseases went further, arguing that Covid should be viewed as a condition that can leave some people with longer-­lasting immune weakness.

Researchers analysed health and blood-test data from about 40,000 patients in China, much of it collected before the pandemic, and then compared it with results from the same individuals after they had been infected with Covid.

This allowed them to track how immune systems changed over time, rather than relying on a single snapshot.

They found that several key components of immune function – including cells involved in fighting infections – were still depleted months after infection.

The effects were most pronounced in men, older adults and people with heart disease.

The authors concluded that, in some cases, Covid may leave the immune system slower to recover, potentially increasing vulnerability to other infections long after the illness has passed.

Since publishing his BMJ article, Nick Tsergas has argued that we need to move beyond what he calls a 'false binary' between those suffering long Covid – the estimated 1.9 million Britons left with ongoing symptoms such as breathlessness, exhaustion and brain fog months or even years after infection – and everyone else.

Instead, he suggests Covid's effects may exist on a spectrum – meaning some people are left severely affected, while others appear to recover, but with more subtle changes that only become apparent over time.

It is a theory that may hold some weight, says Professor Danny Altmann, an immunologist at Imperial College London. 'At the start of the pandemic, the rhetoric from many was that Covid was no different to a cold,' he said.

'The old school of thought with a virus like a cold was that your body fights it off and that's that – but we now know that is not always the case.'

Professor Altmann's research has shown that in some long Covid patients, a 'reservoir' of the virus can remain in the body, driving ongoing symptoms.

'It is not an unreasonable hypothesis that a milder infection could also lead to a subtle impairment of the immune system,' he said.

He also points to research suggesting a rise in autoimmune diagnoses following Covid infection.

A 2023 analysis of health records found that people who had Covid were two to three times more likely to later be diagnosed with autoimmune conditions such as rheumatoid arthritis, lupus and type 1 diabetes. But other studies suggest this increased risk appears lower with later Covid variants.

However, not every expert is convinced Covid has permanently damaged our immune systems.

'I do not believe the theory that our immune systems have been permanently impaired by Covid, although it is plausible,' said Professor Paul Hunter, an infectious disease specialist.

'In a population where the vast majority have been infected, it is extremely difficult to produce high-quality studies with a true control group. It may be the case – but we may never be able to prove it definitively.'

Heightened awareness may also play a role. 'We have all become more conscious of how we feel,' said Professor Altmann. 'And that isn't necessarily a bad thing.'

I had Covid eight times... then all my hair fell out 

For Lydia Morley the theory feels personal. She believes her diagnosis of alopecia was triggered after catching Covid eight times.

‘I think after having it so many times, my immune system has just been dampened and dampened,’ says Lydia, 24.

‘I had Covid at the end of November 2023 for the eighth time, and then I started seeing my hair falling out after that.

‘Whenever I brushed my hair, I’d have proper clumps come out. It was getting to the point where it was strange.’

Doctors told Lydia that Covid could be a factor – but not the only one.

‘Alopecia is one of those conditions where they don’t always know exactly why it happens,’ says Lydia, from Newport, South Wales.

‘They said Covid could be part of it, but it could also be a million other things too.’

Within five months of her diagnosis she lost about 80 per cent of her hair and says she barely recognised herself. She adds: ‘I’m a very outgoing person, and it really takes that away from you. People don’t realise how much of their identity is tied up in how they look.’