Doctors have warned thousands of people across the UK are suffering from norovirus, which causes sudden vomiting and diarrhoea.
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.
Why are rates of norovirus so high this year?
The number of cases fluctuates from year to year and those reported to a laboratory are just the tip of the iceberg, as most people do not visit the doctor.
Several factors are thought to have caused the increase in cases reported this winter.
The norovirus season started unusually early compared to other years, although it is unclear why this is.
There is also thought to be an increased level of awareness of the infection and symptoms by both the public and doctors, leading to identification of more cases.
The Health Protection Agency also say tests to diagnose the virus in the laboratory have got better.
Winter 2002 was the most severe season recorded to date and was associated with the emergence of a new virus strain.