One in five people in Scotland may have been hit by norovirus in the past year, according to new figures.
The high number of cases has led to hospitals across the country having to close wards to new admissions.
A total of 2,824 cases of norovirus - also known as the winter vomiting bug - were confirmed by laboratories in Scotland in the past 12 months.
For each confirmed case, scientists believe hundreds more people could have been affected.
Over the past five years estimated cases have risen by an average of about 25%.
Fourteen hospitals across the country have been affected by norovirus in the past week alone, with 18 wards being closed to new admissions.
Experts say they are not sure why this year has seen such a high number of cases, but it could be the effects of the weather and the ability of the virus to change and adapt, which stops patients becoming immune.
Health Protection Scotland consultant Dr John Cowden told BBC Radio Scotland that research suggested that for every one person recorded as having the virus there are hundreds more cases in the community that go unrecorded.
He said: "There was a study done some years ago to try to find out by what you multiply the national statistics to find out what is really happening in the population.
"Norovirus is usually a very mild disease, most people won't go to their doctor.
"It has been estimated that you need to multiply our laboratory confirmed cases by about 400 to say what is actually happening in the population."
That calculation would take the number of estimated cases of norovirus across Scotland in 2012 to more than one million.
Dr Cowden said the best way to protect yourself from the bug could be summed up in three words - "scrupulous personal hygiene".
He said: "This is an entirely human germ. It passes from person to person. It is not like salmonella or E. coli that you can get from animals.
"There are no guarantees and this is a highly infectious germ. So yes you can protect yourself, but you can't guarantee that you will get through the season without an episode."
Nausea followed by vomiting and diarrhoea are the main symptoms of norovirus.
Those who catch the bug are advised to have plenty of non-alcoholic, non-milky drinks and also take oral rehydration solutions.
Anyone infected should ensure their hands are clean, to prevent the infection spreading, and they should not prepare food for others, particularly babies and the elderly.
Scotland's chief medical officer Sir Harry Burns added: "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."