As many as one in five Scots may have had the winter vomiting bug last year, new figures suggest.
Exactly 2,824 cases of norovirus were confirmed in the first 51 weeks of 2012, according to Health Protection Scotland (HPS).
But experts believe that for every confirmed case, there could be between 239 and 346 unrecorded cases.
The Health Protection Agency (HPA) in England and Wales states that for every reported case, an estimated 288 are not flagged up.
Using the HPA figure, that suggests 813,312 people could have been hit by norovirus in Scotland in the first 51 weeks of 2012, around 15% of the population.
In the worst-case scenario, the number for the whole year could be as much as a million, that is 19% or about one in 5 of the population.
Dr John Cowden, a health protection consultant at HPS, explained the reason for such an apparently large number of unrecorded cases.
"Almost everyone affected with norovirus is only mildly ill and will not visit a health professional. If a health professional does see them, they will not necessarily take a specimen. Even if doctors do take a sample, laboratory testing is not 100% sensitive," he said.
The 2012 statistics reveal a 32% increase on the average number of cases recorded annually in the first 51 weeks of the last five years, Dr Cowden said.
Last year's figure marks a 75% increase on the 2011 level although experts stress that was the lowest recorded annual figure since 2005.
Nausea followed by vomiting and diarrhoea are the main symptoms of norovirus.
Those who catch the bug are advised to drink plenty of non-alcoholic, non-milky drinks and also take oral rehydration solutions.
Those infected should ensure their hands are clean to prevent the infection from spreading and should not prepare food for others, particularly babies and elderly people.
Chief Medical Officer Sir Harry Burns said: "Norovirus, or the winter vomiting bug, can spread quickly where people are living or working in close proximity. The virus is highly infectious and unpleasant but, fortunately, most people make a full recovery with no complications.
"Rates of norovirus fluctuate from year to year with occasional spikes. The Scottish Government and health boards monitor norovirus outbreaks closely throughout the year, taking every step to minimise its impact on people who use and work in our hospitals.
"There are simple steps that everyone can take to prevent the spread of the virus, including washing your hands properly."