Brits are being warned to stay alert this winter as a highly contagious virus is doing the rounds, leaving many people unwell. While some of us assume a sore throat or a fever is just the flu, COVIDor a bad cold, health experts say something else entirely.

"Respiratory viruses circulate all year, but during winter, several circulate at the same time," said the UK Health Security Agency (UKHSA) in a post on X. "[We] explain the signs and symptoms of the most common winter viruses to help you pin down which one you might have."

One of the viruses, known as adenovirus, causes flu-like symptoms and is especially common in the colder months when people spend more time indoors. Medical professionals say cases rise sharply every winter, with children, older adults, and those with weakened immune systems most at risk.

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The UKHSA explained more in a blog post: "Adenoviruses are a group of viruses that cause a range of infections [and] can occur at any time of year. While adenovirus infections are most common in babies and young children, people of any age can be affected."

It often starts off mildly which is why people often ignore it. But in the more vulnerable groups, it can lead to severe chest infections, pneumonia, and in severe cases, death. Experts say people who say they're suffering with a 'mystery winter bug' could actually be dealing with adenovirus, and it's important to raise awareness of the illness before it's too late.

Adenovirus symptoms to look out for

Adenovirus has four flu-like symptoms. These include a high temperature or fever, which may come on suddenly and linger for several days, and a persistent sore throat that feels more painful than a typical cold.

A cough or breathing difficulties are also common, ranging from a dry cough to chest tightness, while a runny or blocked nose often appears early on. Some people may also experience red, sore eyes (conjunctivitis), headaches, body aches or stomach upset such as diarrhoea and vomiting.

Treatment and how to protect yourself

Most healthy adults recover within one to two weeks, but adenovirus can be far more serious for babies, the elderly and people with underlying health conditions. In these groups it can cause bronchitis, pneumonia or severe dehydration, sometimes requiring hospital treatment.

There is no specific cure for adenovirus. Treatment focuses on rest, fluids and pain relief such as paracetamol. Antibiotics do not work because the infection is viral.

The NHS advise seeking urgent medical help if symptoms worsen, breathing becomes difficult, or a high fever does not come down. Parents are also urged to get advice if young children become unusually drowsy or stop drinking fluids.

Adenovirus spreads easily through coughs, sneezes and contaminated surfaces, and can survive for long periods outside the body. Regular handwashing, cleaning surfaces and staying home when unwell are key to stopping its spread.

Health experts say don’t brush off flu-like symptoms this winter – especially if you or someone close to you is vulnerable. What feels like a simple cold could be something far more serious.