AVIRULENT stomach infection known as the winter vomiting virus is becoming increasingly prevalent during warmer months and has now reached unprecedented levels across Scotland.

Health officials believe as many as half a million Scots were affected by the highly infectious norovirus at some point during the first five months of this year, although the figure could be higher.

The number of reported infections of the virus in the first 20 weeks of 2006 already exceeds the total number of infections reported for last year, and is 700 higher than in the equivalent period of 2005.

For every incident, say infection monitors, up to hundreds of Scots are affected, as many people who contract the bug do not seek medical advice.

Though common in hospital wards and care homes, the rise comes in the wake of several unseasonal outbreaks of the disease on ferries and cruise ships in recent weeks, which are increasingly a prime site for widespread infection.

The medical profession remains unable to account for the significant upturn in sickness, although it is believed in some circles that a new virulent strain of norovirus is to blame.

Yesterday, a ferry service between Stornoway and Ullapool was suspended after eight crew members fell ill, reporting symptoms of the highly contagious bug.

Ferry operator Caledonian MacBrayne was forced to clean the vessel and arrange for a new crew to come on board. Samples have been sent to the Western Isles public health department for testing.

Only last week, a cruise ship with more than 1700 Britons aboard was forced to abandon a seven-night European tour after passengers suffered the virus.

In April, the MV Caledonian Isles was taken out of service between Ardrossan and Brodick for two days after an outbreak of the infection.

In its latest weekly surveillance report, Health Protection Scotland reported there had been 1755 confirmed cases of norovirus in the first 20 weeks of 2006, compared with 1080 cases in the equivalent period last year.

Dr John Cowden, consultant epidemiologist at the agency, said: "For the number of infections to go up by around 75per cent is a big increase.

"Outbreaks tend to happen in winter in establishments such as hospitals, but we are seeing an increase of infections in summer months in tourist settings such as cruises, coach trips and long-stay hotels, places where large groups of people are in close proximity."

The infection, characterised by nausea, sickness, diarrhoea and flu-like symptoms, usually lasts for around 24 hours and, unless an individual already has another serious disease, it is not considered fatal. Those infected are advised to take in fluids to prevent dehydration, but antibiotics have little effect.