Norovirus cases in England have risen “significantly”, with levels among over-65s the highest in more than 10 years, figures show.
The majority of outbreaks are in care homes, but there have also been increases in schools and hospitals.
Health experts urged people with symptoms to stay at home and not to return to work, or send sick children to school, until 48 hours after symptoms have cleared.
Cases of norovirus reported by laboratories are currently running 66% above the average for the past five seasons, according to the UK Health Security Agency (UKHSA).
But because only a small number of people seek medical help for the virus, prevalence within the wider community is likely to be much higher.
The biggest increase in laboratory-confirmed norovirus is among people aged 65 and over.
While high numbers of cases in this age group can be expected at this time of year, current levels haven’t been seen in more than a decade, the UKHSA said.
Norovirus is the most common infectious cause of vomiting and diarrhoea.
It spreads easily through contact with someone who has the virus or with contaminated surfaces.
While most people make a full recovery within two or three days, the virus can lead to dehydration, especially among the very young, elderly or those with weakened immune systems.
“Most reported cases are in the over-65s and we’re also seeing a rise in reported outbreaks, particularly in care home settings.
“Please stay at home if you are experiencing norovirus symptoms and do not 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 soap and warm water is best.”
Hospital cases of norovirus in England have nearly doubled week-on-week, NHS data shows.
At this point last year the average stood at 302.
Professor Stephen Powis, NHS medical director for England, said hospital cases of norovirus have risen “significantly” in line with what is being seen in the community and in care homes.
“It is a really unpleasant illness to catch, but for the vast majority of people it will usually pass in a couple of days, and self-treating at home is the best way to help yourself and avoid putting others at risk,” he added.