Why you’re ill again after having ‘super flu’ as ‘mystery’ illness hits UK

There's a reason why you might be reading this with a runny nose and scratchy throat, with a sickness that has felt like it's carried on forever.

You're not alone, with many Brits falling ill with a 'mystery virus' in the early days of 2026, which has come after the annual winter flu.

Some of the symptoms you may be experiencing could include coughing, sneezing, and stomach problems - and it's nothing to do with the dreaded 'super flu' which did the rounds earlier this winter.

The NHS warned about this viral infection for months on end, even suggesting that people could die in what is the worst flu season in decades.

It's believed that while this mutated from an illness in the summer, the new 'mystery' illness spreads easily and has been linked with several common respiratory viruses.

The winter season typically encourages the spread of a number of illnesses (Getty Stock Image)

What is the 'super flu'?

While the flu already comes with symptoms such as chest pain, vomiting and diarrhoea, doctors have highlighted that vomiting yellow bile is a telltale sign of the so-called 'super flu'.

It's the key symptom to look out for, according to experts, with yellow bile being the liquid produced in the liver that breaks down fats in the small intestine.

This sounds grim, but Dr Simon Clarke, an infectious disease expert at the University of Reading, said that it can happen when 'someone is vomiting on an empty stomach'.

Others suggest that this horrific symptom can be due to the flu itself suppressing the appetite, causing the vomiting of yellow bile.

What is the 'mystery virus'?

It's called the Adenovirus, and according to Ian Budd, a pharmacist at Chemist4U, it can be 'more likely to cause fever and conjunctivitis, and symptoms can last longer' than a cold.

Speaking to The Mirror, he pointed out why it's being caught after the 'winter flu', saying: "With COVID-19 restrictions lifted, and fewer people catching other viruses over the past few years, immunity in the community may be lower, leading to people becoming more susceptible."

According to the expert, it lasts longer than the regular cold and is 'more likely to cause fever and conjunctivitis', while it can also cause acute bronchitis, pneumonia, pink eye, and acute gastroenteritis, in addition to regular flu symptoms.

Adenovirus can be spread from one person to another through close physical contact and germs travelling through the air.

Vomiting yellow bile is a symptom of the 'super flu' (Getty Stock Image)

What's the difference between the 'super flu' and Adenovirus?

Adenovirus affects the upper and lower respiratory tracts, as well as the eyes, stomach, and bladder. It can be identified by symptoms often seen in flu cases, which means doctors or patients themselves can miss it.

Vomiting yellow bile is the way to identify the 'super flu', but there are bigger signs to look out for when it comes to Adenovirus, which can also involve throwing up.

Issues surrounding the eye could come across as conjunctivitis or pink eye, which could be itchy or watery, in addition to blurry vision and watery discharge.

More serious cases of adenovirus can include constipation, diarrhoea, vomiting and tummy pain, which don't always occur with the flu.

Contracting a painful urinary tract infection (UTI) can also be the result of catching germs. There are currently no approved medicines or treatments for Adenovirus.