Norovirus levels across the UK are "the highest we have seen at this time of year in over a decade", a doctor at the UK Health Security Agency (UKHSA) has warned.
Cases of the highly infectious stomach bug have surged in recent weeks, with hundreds of NHS hospital beds filled by people with symptoms.
Lab reports of norovirus are more than double the five-season average before the coronavirus pandemic, according to the UKHSA.
It said reporting has increased across all age groups, most notably those aged 65 and older.
The virus is known as the "winter vomiting bug" because it is notorious for spreading rapidly during the colder months.
The NHS says symptoms include feeling or being sick and diarrhoea, and those infected may also experience a high temperature, headaches and aching arms and legs.
Most people will fully recover within two to three days, but those with the illness are urged to drink plenty of fluids to prevent dehydration, especially the very young, elderly or those with weakened immune systems who are most at risk.
While the majority of outbreaks continue to be reported in care homes, cases in hospitals are also increasing, but remain below the five-season average.
Outbreaks reported in care homes increased from 47 in the week beginning 6 February, to 54 in the week beginning 13 February, representing the highest number of outbreaks reported in a week so far this season.
'Please stay at home if you have symptoms'
Dr Lesley Larkin, surveillance lead of gastrointestinal infections and food safety at UKHSA, said the increase represents "the highest (levels) we have seen at this time of year" in more than 10 years.
"Most reported cases are in the over 65s and we're also seeing an increase in reported outbreaks, particularly in care home settings," Dr Larkin added.
"Please stay at home if you are experiencing norovirus symptoms and don't return to work (particularly if you work with vulnerable people or food) or send sick children to school or nursery until 48 hours after symptoms have cleared.
"If you have a loved one in a care home or hospital, please avoid visiting until 48 hours after symptoms have cleared.
"Regular hand washing is really important to help stop the spread of this bug, but remember, alcohol gels do not kill off norovirus so washing with soap and warm water is best."
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