Thousands of people across Britain are affected by it every winter - and last week it is even believed it may have caused its first fatality, with the death of Berkshire schoolgirl Jodie Mettrick.

But what exactly is the dreaded Norwalk-like virus, also known as the winter vomiting bug or Norovirus? How does it spread so quickly, and how can you try to avoid it?

Vicki Parkin, senior infection control nurse for York Hospitals NHS Trust, says the key is in containing the bug - which spreads easily through air, food, water and people.

"It's community-acquired," she explained. "Someone introduces it into the hospital - it could be a patient, a visitor or a member of staff.

"You have to try and isolate those symptomatic patients, so they will recover themselves. If you don't try and stop the spread, it will just go on and on. It's very difficult to control. We keep the entrances and exits to the wards closed.

"It can be very, very debilitating for the elderly."

Norovirus has recently attacked Selby and York Hospitals, with health managers shutting off wards to try and contain the sickness.

Latest to be hit were beds at Selby War Memorial Hospital, where both wards were shut last week after elderly patients were struck down with the virus.

That came after seven wards at York Hospital were closed to new patients earlier this month because of the bug, with visitors only let in under caution.

This week, the hospital was left with just one shut ward, while health chiefs in Selby hoped that one of its wards could be reopened this weekend.

But if the effects of Norovirus are so debilitating in community settings, from schools to hospitals, what is Vicki's advice to anyone who is struck down with the bug?

"If you have it, and you're not an inpatient and you don't have to come into hospital, then don't," she says.

"Drink plenty and keep yourself comfortable."

What is Norovirus?

It's the most common cause of infectious gastroenteritis - an inflamed stomach and intestine - in England and Wales. Often referred to as winter vomiting disease because of its seasonality and typical symptoms.

Where does it most commonly break out?

In community, semi-closed environments like hospitals, nursing homes, schools and cruise ships.

Symptoms?

These begin about 12 to 48 hours after infection, and last for between 12 and 60 hours. Look out for a sudden nausea followed by projectile vomiting and diarrhoea. In some people, it may cause a raised temperature, headaches and aching limbs. Most people recover within one or two days, but others - usually the very young or elderly - can get dehydrated and require hospital treatment. Health Protection Agency advice says those who have been infected should stay isolated for up to 48 hours after the symptoms have gone.

How does it spread?

Contaminated food, especially shellfish, and water. It can be passed from person to person, and through the air. Toilet seats can also be contaminated. The virus often causes outbreaks because it spreads easily and can survive for many days.

What to do if you get it?

There is no real answer other than letting the virus stay its course. Drink plenty of fluid to avoid dehydration, and keep comfortable.

How do you stop it spreading?

With good hygiene, such as disinfecting contaminated areas and washing hands. Avoid preparing food for up to three days after symptoms have gone.

How common is the virus?

The Health Protection Agency sees between 130 and 250 outbreaks each year. It is estimated it affects up to a million people in the UK every year.

Source: The Health Protection Agency

Updated: 09:03 Friday, February 17, 2006