With the latest figures suggesting that up to 1.12 million people could be affected by an outbreak of norovirus in the UK, Channel 4 News looks at the virus and why it is so contagious.
What are the symptoms of norovirus?
Noroviruses are the most common cause of stomach bugs, or gastroenteritis, in the UK.
Symptoms include a sudden onset of vomiting and/or diarrhoea. Some people may have a temperature, headache and stomach cramps.
The illness usually resolves in one or two days and there are no long-term effects.
How many have been affected this winter?
It is still early days, but figures published by the Health Protection Agency (HPA) on 02 January show there have been 3,877 confirmed reports of norovirus so far during the 2012/13 season, but the agency added that for every reported case there are likely to be a further 288 unreported sufferers.
Laboratory-confirmed reports represent only a small proportion of the actual amount of norovirus activity in the community, because the vast majority of affected people do not access health care services as a result of their illness. This means that potentially 1.12 million people could be affected by the stomach bug - a 72 per cent increase compared with the same period in the 2011/12 season.
The bulk of cases of norovirus usually occur between January and March.
A spokesman for the HPA said there was no indication as to why there are more cases of norovirus this year than last.
How do you catch norovirus?
Norovirus is highly contagious and can be transmitted by contact with an infected person, by contact with contaminated surfaces or objects, or by consuming contaminated food or water.
The virus spreads rapidly in closed environments such as hospitals, schools and care homes. This is why it has hit passengers cruising the Baltic on board the Oriana so hard, with passengers on the liner confined to their rooms and warned not to leave the ship. Channel 4 News was told the final figure for those affected reached over 400.
According to the HPA, once a person has contracted norovirus they are immune to that particular strain for between three to six months afterwards. However, there are many different strains of norovirus and they would not be immune to other variations of the bug.
John Harris, an expert in norovirus at the HPA, says there are measures you can take to prevent norovirus: "People should be vigilant in their hygiene, and we would like to remind anyone who has typical symptoms suggestive of norovirus infection to avoid visiting friends or relatives in hospital or care homes.
"Norovirus infection in hospitals is very disruptive as it can lead to ward closures."
What should you do if you have norovirus?
Norovirus infection is a self-limiting illness, and patients will recover naturally without treatment. Anyone who suspects they have the norovirus are advised not visit their GP surgery or local A&E unit, but to instead get advice from the online service NHS Direct.
To stop it spreading, people experiencing the symptoms are advised to wash hands thoroughly and regularly at all times, but particularly after using the toilet and before eating.
"Having a norovirus infection is very unpleasant but it is short-lived and most people will fully recover in a couple of days," said Mr Harris.
"Make sure that you or anyone you are caring for takes plenty of fluids to avoid dehydration. Over-the-counter medicines can also be useful in reducing headaches and other aches and pains".
Why is norovirus more common in winter?
The norovirus can be caught at any time of year, but is known as "the winter vomiting bug". Channel 4 News Science Editor Tom Clarke says it did not get that name by accident. "Though it occurs year round widespread outbreaks of the virus are most common in cold weather.
"Scientists aren't precisely sure why the virus prefers the winter - but it is thought the bug finds it easier to spread. In winter people tend to spend more time indoors in close proximity to one another, increasing the chance an infected person passes it on to a new host.
"Also, viruses like norovirus that are spread by contact may survive for longer outside of the body during winter. Lower temperatures and lower levels of sunlight mean viruses may break down more slowly on contaminated surfaces like doorhandles taps and toilet flushes."
Read more: Norovirus cases up by 100,000 over Christmas