Wards in hospitals across the UK have been closed to visitors to try to prevent the spread of winter vomiting bug norovirus, which causes sudden vomiting and diarrhoea. What is it?
What is it?
Noroviruses are a group of viruses that are the most common cause of stomach bugs in the UK, affecting all ages.
It is estimated that between 600,000 and a million people in the UK become infected each year. Noroviruses are also sometimes known as "winter vomiting viruses" or "Norwalk-like viruses".
What are the symptoms?
Around 12 to 48 hours after becoming infected, the virus causes sudden onset of nausea followed by projectile vomiting and watery diarrhoea.
Some people may have a fever, headaches and aching limbs - often leading people to call the illness "stomach flu".
The illness is not generally dangerous and most people make a full recovery within one to two days.
However, the very old and very young risk becoming dehydrated which may require hospital treatment.
How does it spread?
The virus is easily transmitted from one person to another by contact with an infected person or through contaminated food or drink or touching contaminated surfaces or objects.
It is able to survive in the environment for many days and outbreaks tend to affect more than 50% of susceptible people.
Anyone is susceptible as immunity to the virus is not long-lasting.
Outbreaks commonly occur in hospitals, nursing homes, schools and on cruise ships.
It can be difficult to control because it is so easily spread but the best way is to disinfect contaminated areas, washing hands and to avoid handling food.
Also, those who have been infected should be isolated for 48 hours after their symptoms have gone away.
What is the treatment?
There is no treatment other than to let the illness run its course.
Those infected should drink plenty of fluids to prevent dehydration and practise good hygiene to reduce the risk of the illness spreading.
'Vomiting Larry' is busy being sick over and over again in an experiment to test just how far the winter vomiting bug can travel when it makes you ill.
Lucky for Larry, he is not a constantly retching human - but a simulated vomiting system that shows the virus can travel an impressive 3m (9.8ft) in a projectile episode, according to his creators at the Health and Safety Laboratory.
The winter vomiting bug (norovirus) has been responsible for an estimated 880,000 cases of vomiting and diarrhoea in the UK since the summer.
It is a hardy virus that clearly spreads with ease - one of the few infections you really can catch from a toilet seat, or even from the air in the bathroom if an infected person has recently pulled the flush.
Luckily most people make a full recovery in a few days, but for anyone who is already vulnerable - people who are already unwell or in their later years for example - it can pose a serious threat.
It was first noticed when 150 children at the Norwalk-Bronson Elementary School in Ohio were all struck down with the bug in 1968.
This 40-year-old incident is now eternalised in medical history, making up the first part of the virus's name.
And unsuspecting groups of students continue to be knocked down by its tenacious grip on the small intestine.
A girls' football team from Canada was taken ill after a team-mate developed the illness - but she had had no contact with them.
The culprit was a grocery bag in the corner of the bathroom she had used.
One in 20 people in the UK suffer from norovirus each year
Aerosolised particles of the virus landed on the grocery bag, and spread to members of the team who touched the bag or ate the packaged crisps and cookies inside it, according to research published in the Journal of Infectious Diseases.
And it can stick around for a long time.
It is widely accepted that it can last for two weeks on hard surfaces, says Professor Ian Goodfellow of the University of Cambridge.
It is not only its ability to travel so far or survive on everyday surfaces that makes it, in viral terms, "the ideal infectious agent", says Dr Aron Hall, from the Centers for Disease Control and Prevention in the US.
'Rapid reproduction'
As few as 18 viral particles can infect a new person, but there are often thousands of particles present in each drop of vomit.
And it is able to reproduce at a rapid speed - thousands of times more quickly than humans.
The fact it makes most of us only mildly or moderately ill is all part of its high-achieving strategy.
If it made people more seriously ill, or even killed them, it would not be able to spread so well.
"From an evolutionary stand point if you kill your host you are not going to have anywhere to live, so keeping your host alive has its advantages," says Dr Hall.
All these factors make it very difficult to stop in its tracks.
That is, apart from the good old fashioned method of washing your hands with soap and water.
You may also have a fever, headache and stomach cramps
Over-the-counter medicines can be useful in treating headaches and other aches and pains
Do not visit your GP surgery or A&E unit if you have it
If symptoms persist for more than three or four days, or if you have a serious illness, seek medical attention through contact with your GP
Wash hands thoroughly, particularly after using the toilet and before eating
Clean hard surfaces with detergent followed by disinfection with a bleach solution, paying particular attention to the toilet and surrounding area
Source: Health Protection Agency
According to a study of seven outbreaks at an international scout jamboree, each boy scout who had the infection passed it on to 14 others until enhanced hygiene measures were imposed.
This included separate bathrooms for infected people and strict hand-washing regimes.
The number of infections passed on per person then went down to two - but still not enough to stop it from spreading completely.
The virus also manages to evade many commonly used alcohol gels and some domestic cleaning products.
You are safe with thoroughly cooked food, but it can escape freezing and mild heating.
Scientists have spent many years trying to find a way to stop it from closing hospital wards, leaving offices short-staffed in winter months and causing chaos on cruise ships.
Legend has it that the first of the now many attempts to study the virus involved volunteers who drank the filtered diarrhoea of people who had the infection.
Study designs may have moved on a lot since then, but we are still far from finding a cure.
One of the reasons is that no one has been able to persuade the virus to grow in a lab, says Prof Ian Goodfellow:
"In my lab we are trying to understand how these viruses work, with the overall aim of trying to identify a drug that will prevent infection and control outbreaks when they do occur," he says.
But no one has yet pinned down why it is so elusive. And until a vaccine or cure is found it is likely to continue to put some people off their turkey this Christmas.
The Health Protection Agency estimates there have been about 880,000 cases in England and Wales since the summer, 83% more than in the same period last year.
But the HPA says these reported cases are the tip of the iceberg and for each one, there will be around 288 that go unreported.
Health Protection Scotland has also reported a rise in cases.
Early increase
The figures also show there were 61 outbreaks of norovirus in hospitals in the fortnight up to December 16 - almost double the number in the same period last year when there were 35.
Norovirus is highly contagious and can be transmitted by contact with an infected person, contaminated surfaces or objects or through consuming contaminated food or water.
It spreads rapidly in closed environments such as hospitals, schools and care homes.
The norovirus "year" - the date from which experts start to count cases - begins in July and runs to the following June.
The increase in norovirus cases is occurring earlier than normal - but experts are unsure why this is happening.
And these figures are a small fraction of the number of confirmed cases seen during the usual peak period of January to March.
However the HPA stresses norovirus is unpredictable, and no two years are the same.
Laboratory confirmed reports represent a small proportion of the actual number of cases because most people do not see a doctor - and therefore their case is not recorded.
'Short-lived'
John Harris, an expert in norovirus at the HPA said: "The number of laboratory confirmed cases has risen again, following the drop in the number we reported last week.
"This is typical of the norovirus season where the number of laboratory reports fluctuates between October and April with the bulk of cases usually occurring between January and March.
"Norovirus is very contagious so we would urge anyone who thinks they may be unwell with norovirus to stay at home and stay away from hospitals and care homes.
"The infection is short-lived although it is very unpleasant while you are unwell.
"Most people will not need to go to see their doctor and will recover in a couple of days. It is important to take plenty of fluids to avoid dehydration."
A Department of Health spokesman said: "The NHS is well prepared for the increase in winter related health problems which are typical at this time of year.
"Our weekly published figures show the number of beds closed across the NHS due to norovirus symptoms is around 2%.
"This compares to 2.9% of beds that were closed during the peak of norovirus cases last winter."
The latest Scottish figures, published on 5 December, showed 2,016 laboratory reports so far this season - up 46% on 2011 figures.
The Health Protection Agency says there has been a 72% increase in cases on last year
