Hospitals in England closed more than 1,100 beds in the past week because of the winter vomiting bug norovirus.
NHS officials are urging people with the virus not to go to work and parents not to send children with it to school, until 48 hours after symptoms have gone.
Those who need advice should use the NHS website or free phone line rather than going to their GP or hospital.
Data shows higher than average levels of norovirus in November.
One of the most common stomach bugs in the UK, it can be spread very easily in the community.
Young children, elderly people and those with pre-existing medical conditions are most at risk if they catch it.
There are more than 100,000 hospital beds in England, and 1% have been affected by norovirus – but this situation has happened before.
How to spot the signs of norovirus:
The main symptoms of norovirus are typically:
suddenly feeling sick
Some people also have a slight fever, headaches, painful stomach cramps and aching limbs.
The symptoms appear one to two days after people become infected and typically last for up to two or three days.
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Five ways to limit the spread of norovirus:
Stay at home if you are experiencing symptoms, do not return to work or send children to school until 48 hours after symptoms have cleared and avoid visiting elderly or poorly relatives, particularly if they are in hospital
Wash your hands frequently and thoroughly with soap and warm water. Alcohol hand gels don’t kill norovirus
Use a bleach-based household cleaner or a combination of bleach and hot water to disinfect household surfaces and commonly used objects such as toilets, taps, telephones, door handles and kitchen surfaces
If you are ill, avoid cooking and helping prepare meals for others
Wash any contaminated clothing or bedding using detergent and at 60C and if possible wear disposable gloves to handle contaminated items
Outbreaks of norovirus are affecting schools as well as hospitals.
Four hospital wards were closed to visitors at a hospital in Reading yesterday and, earlier this week, a primary school in Leicester and another in Liverpool were closed after a suspected outbreak of norovirus.
NHS England medical director Prof Stephen Powis said instances like these were “likely to rise over the coming weeks”.
“It’s 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.
“Crucially, if you’re experiencing norovirus symptoms, it’s important that you don’t return to work or school for 48 hours after they clear – and avoid visiting elderly or ill friends and relatives – to avoid spreading it to other people.”