Public health experts are warning of a surge in the vomiting bug norovirus as COVID-19 restrictions ease.
The highly infectious norovirus, which causes vomiting and diarrhoea, normally peaks in winter months.
But cases of the bug have recently been increasing across England.
Public Health England (PHE) has warned: "It is possible that unusual or out-of-season increases could be seen in the coming months following further easing of COVID-19 control measures."
Outbreaks have been particularly concentrated in nursery and childcare facilities with far more than expected in summer months, PHE said.
In the past five weeks, 154 outbreaks have been notified, compared with an average of 53 outbreaks for the same time period in the previous five years.
PHE said while young children were affected, there has also been a rise in all age groups.
Professor Saheer Gharbia, deputy director of PHE's National Infection Service, said: "Norovirus, commonly known as the winter vomiting bug, has been at lower levels than normal throughout the pandemic with less opportunity to spread between people in the community, but as restrictions have eased we have seen an increase in cases across all age groups.
"Symptoms include sudden onset of nausea, projectile vomiting and diarrhoea but can also include a high temperature, abdominal pain and aching limbs.
"Stay at home if you are experiencing norovirus symptoms and do not return to work or send children to school or nursery until 48 hours after symptoms have cleared.
"As with COVID-19, hand washing is really important to help stop the spread of this bug, but remember, unlike for COVID-19, alcohol gels do not kill off norovirus so soap and water is best."
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Norovirus is easily transmitted through contact with infected people or contaminated surfaces.
A bleach-based household cleaner or a combination of bleach and hot water should be used to disinfect household surfaces that may be contaminated, as well as commonly used objects such as toilets, taps, telephones, door handles and kitchen surfaces.
Those who are sick should avoid cooking and helping prepare meals for others until 48 hours after symptoms have stopped, as norovirus can be spread through contaminated food.
The bedding and clothing of those affected should also be washed at 60C, with disposable gloves used to handle contaminated items, PHE said.
Those showing symptoms should avoid visiting their GP or hospital, but can contact NHS 111 or talk to their GP by phone if they are worried.
Dr Nick Scriven, immediate past president of the Society for Acute Medicine, said: "We are now just days away from the lifting of the remaining restrictions in England and our NHS is under great strain.
"This has been compounded today by the warning that cases of norovirus, commonly known as the winter vomiting bug, have now reached pre-pandemic levels in summer.
"Considering the impact this has when it makes its way into hospitals - bed closures, infecting seriously unwell people and staff absence - it is frankly very worrying.
"At the moment, clinicians across the country are asking themselves exactly what the government is thinking given the fact healthcare is being ravaged."