Hospitals in England have been forced to close more than 1,100 beds in the last week to stop the spread of norovirus.
The NHS has urged people who feel the symptoms of norovirus, also known as the winter vomiting bug, to take precautions to avoid infecting others and putting a strain on public services - for example staying at home and washing hands thoroughly with soap and water.
It comes after Public Health England (PHE) found the number of norovirus cases in hospitals for mid-November was 28% higher than the average number of cases in the same period over the last five years.
Almost double the number of hospital beds have also been closed every day over the last week than at the same time last year as trusts try to stop the bug spreading between patients.
What is norovirus?
Norovirus causes vomiting, nausea and diarrhoea and usually lasts about two days, but can last longer for the young, the elderly, and those with weak immune systems.
Symptoms can also include high temperatures, headaches, and aching joints.
It's highly contagious and can be spread by touching infected hands, food, or surfaces.
Most common alcohol antiseptics don't kill the virus - only washing your hands with soap and water will reduce the risk of infection.
Official NHS advice to those affected is to drink plenty of fluids and stay at home for at least 48 hours after symptoms have passed to avoid spreading the virus.
NHS medical director Professor Stephen Powis said in the majority of cases, people with norovirus do not need to visit their GP.
“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," he said.
Instead, Prof Powis said those affected could call NHS 111 or contact their GP by phone for advice.
Why have beds been closed?
Dr Nick Scriven, a former president of the Society for Acute Medicine, explained that patients cannot be discharged from wards with norovirus into places such as care homes as it could increase the spread of the illness.
He added the bed closures are "a great concern on top of a system already pushed to its limits.
"It is worrying for many reasons, one of which is that it is not a minor illness and can be fatal for some.
"For the system, it closes beds and also slows discharges, as you cannot discharge a patient from an affected ward to a care home.
"All in all, the rise in norovirus and flu is going to be a severe challenge for a system already stressed to the max."
How can it be treated?
Infections don't usually require medical treatment, so the best cure is to wait for the illness to pass.
Experts advise drinking plenty of fluids, preferably water or squash, take paracetamol if necessary, and only eating when you feel you're able to.
They advise against drinking fruit juice or soft drinks, which will only make the diarrhoea worse.
Cleaning commonly-used objects, such as the toilet, taps, telephones, and door handles, and washing clothes and bedding at 60C will help prevent the spread in your home.