A healthy little girl who doted on her cuddly Winnie-the-Pooh bear and adored the Gruffalo, Lorelei Cowmeadow had only ever had the usual colds and coughs all small children get from time to time.

So when the lively toddler seemed 'a bit under the weather' with 'cold-like symptoms' one Sunday last November, her parents Jack and Antonia naturally assumed she had a winter bug.

Indeed, Lorelei went to nursery as normal the next day - and, although she'd needed a dose of Calpol during the day, was dancing exuberantly in front of CBeebies at home come teatime.

'We really thought it was an ordinary evening,' recalls Antonia, 31, a personal assistant. But the couple were abruptly woken in the middle of the night by Lorelei's cry.

She was asking for a bottle and her nappy also needed changing, both of which were 'a bit odd' as she normally slept through the night, they recall.

Jack changed her nappy, while Antonia went to prepare a bottle of milk. Then suddenly everything changed: Lorelei begin to fit in her father's arms.

'Lorelei went from being an articulate little girl - who'd been clearly saying "bottle" and "daddy" to just mumbling, almost grunting, and making strange noises,' says Jack, 31 who works in sales.

When Lorelei Cowmeadow seemed 'a bit under the weather' last November, her parents Jack and Antonia naturally assumed she had a winter bug

He immediately shouted for his wife - it was very clear to them that something was seriously wrong. Both parents wipe away tears at the memory, as Jack recalls: 'Her head was rolling back and her eyes were going backwards.'

Antonia rang 999, while Jack desperately tried to get his daughter to respond.

'It was surreal - only a few hours before she'd been the bubbly, bossy Lorelei we loved - yet now she was clearly critically ill,' says Antonia. 'We just couldn't believe it was happening.'

Within hours, Lorelei was in an induced coma in hospital while doctors ran tests to work out what was wrong. They also gave her antivirals and steroids to fight off any potential infection.

Yet she died three days later, on November 21, 2024 - just three days before her second birthday.

Antonia says: 'Neither of us could take in the magnitude of what happened; the speed she went downhill was beyond belief and within a few hours we were saying goodbye to our little girl.'

Lorelei died after developing encephalitis, a serious neurological condition where the brain becomes inflamed and swells.

It can be caused either by an infection directly invading the brain (known as infectious encephalitis) or through the immune system going into overdrive and mistakenly attacking the brain (autoimmune encephalitis). In Lorelei's case, it was a complication of the flu.

There are around 6,000 cases of encephalitis every year in the UK.

While it can affect anyone, there's an increased risk in younger children - like Lorelei - and older people whose immune systems are less robust, says Professor Benedict Michael, chair of infection neuroscience at Liverpool University.

Between 10 to 20 per cent of cases prove fatal: the vast majority of those who do survive can be left with life-long disabilities, such as memory loss, language and cognitive problems, fatigue, repeated seizures and epilepsy, adds Professor Michael.

Infectious encephalitis is normally caused by a virus - most commonly herpes simplex (the cold sore virus) and varicella zoster (chickenpox), but also (more rarely) flu and measles. 'Any virus has the potential to trigger encephalitis - but not everyone who is infected with these viruses will develop it,' says Dr Ava Easton, a senior research fellow in encephalitis at Liverpool University and chief executive of the charity, Encephalitis International.

'It's not known why one person goes on to develop encephalitis when another doesn't.'

Different viruses invade or damage the brain in different ways, explains Professor Michael.

Herpes simplex and varicella zoster enter the central nervous system by travelling along the nerves - 'these viruses may lie dormant for years and then become reactivated, perhaps due to weakened immunity, and can trigger encephalitis', he says.

It's much more rare for flu to cause encephalitis, as happened to Lorelei - it's thought to occur if the body's immune response is excessively strong, leading it to release chemicals known as cytokines that can cause inflammation and swelling.

As Dr Easton explains: 'The infection causes the brain to swell - but as it's encased in the rigid skull it can't expand, so the tissue becomes damaged.'

Prompt treatment is clearly vital. Yet the condition is often missed because its symptoms are easily mistaken for other conditions, plus those symptoms can start off very mild.

'Symptoms can be flu-like or include headaches, sensitivity to the light, nausea and vomiting - which can seem generic at first and can easily be mistaken for other conditions,' says Professor Michael. 'And once established, encephalitis progresses very rapidly.'

Lorelei died after developing encephalitis, a serious neurological condition where the brain becomes inflamed and swells

There are a number of ways to diagnose it but the critical test is:a lumbar puncture (where the spinal fluid is analysed for white blood cells which indicate inflammation), brain scans and blood tests.

Patients are initially treated with antiviral medication on an aggressive treatment plan. For instance, acyclovir is effective against the herpes simplex and chickenpox viruses - 'it gives an 80 to 90 per cent chance of survival if administered within the first 24 hours', says Professor Michael. Ganciclovir, used to treat immuno-compromised patients with viruses, is also often administered

But there are no specific antivirals for most other viruses, so treatment is targeted at relieving symptoms, using anti-seizure medication, for example.

Autoimmune encephalitis is treated with medicines that calm the immune system, such as corticosteroids.

'In most cases, there is a window of opportunity where early intervention improves survival,' says Professor Michael.

That's why awareness of the condition is so vital - 'otherwise, all too often it leads to missed opportunities', says Dr Easton.

'We want GPs, A&E staff and the public at large to be aware that encephalitis is a possibility, in the same way that people are now aware of the dangers of meningitis or sepsis - because there can often be a vital 24- to 48-hour window to treat them and it can make the difference between life and death.'

Read More My dry eyes and gut symptoms were signs of a little-known disease that mainly strikes women

It's estimated 77 per cent of the public in the UK do not know what the condition is, according to a 2021 YouGov survey.

Tragically, tiny Lorelei's condition deteriorated so rapidly that there was little that could be done. She was initially blue-lighted from her home in Wokingham to the Royal Berkshire Hospital in Reading, where concerned doctors ran a series of tests and took the decision to put her into a drug-induced coma.

She was then transferred to John Radcliffe Hospital in Oxford for more specialist care.

Following an MRI scan, doctors gently told her parents that Lorelei had caught the flu, which had caused her to develop encephalitis - and her swollen brain had been damaged.

Lorelei was given an antiviral medication, 'but that didn't touch the sides', recalls Antonia.

'We were initially told there was a chance she might live but be brain damaged, but it was soon clear that this wasn't the case...'

Family and close friends came to the little girl's bedside to say goodbye over the next couple of days. Jack and Antonia also took the brave decision to donate her kidneys to help a stranger. The couple later learned they had been successfully transplanted into a 40-year-old woman.

'We want some good to come out of this tragedy,' says Jack, simply. Antonia adds: 'Losing Lorelei has left us heartbroken. We want people to be aware of this terrible condition and the devastation it can cause.'

Symptoms to watch for

Both infectious and autoimmune encephalitis can begin with flu-like symptoms or a headache followed by more severe symptoms.

While it's important to be aware of both types, a key difference is the speed with which they can progress, says Professor Benedict Michael, chair of infection neuroscience at Liverpool University: 'While infectious encephalitis can progress rapidly - within hours or days - autoimmune symptoms develop more slowly, over days or even weeks.'

He adds: 'One key thing to look for - which is easily missed - is if a person is not behaving in a way that is normal for them.

'Trust your instincts and if you are concerned about someone, ensure that they seek medical help quickly - if only to rule out encephalitis.' Here are other main signals to look out for:

Infectious encephalitis:

Fever

Sensitivity to light

Neck stiffness

Difficulty moving or speaking

Weakness in arm, leg or face

Sensory changes

Seizures

Drowsiness

Autoimmune encephalitis:

Confusion

Psychosis

Seizures

Difficulty moving

Hallucinations

Memory loss

encephalitis.info

Within hours of coming down with flu, little Lorelei was dead