Figures published by Health Protection Scotland (HPS) reveal there were 2824 reports of norovirus in Scotland up to week 51 in 2012, up 30% on the average over the previous five years.
Based on the findings of a recent study which suggested that for every norovirus case confirmed by a lab there were between 239 and 346 in the community, HPS estimates this means up to one million Scots – one in five – could have been affected.
HPS consultant Dr John Cowden said: "As 2824 times 346 is 977,104, and as this is not an exact science, and week 52's figures are not yet available, it means as many as one million people, ie 19% or about one in five of the population of Scotland, may have been infected."
HPS observed last year that laboratory reports of norovirus had "increased markedly" around week 43, in late October, when it confirmed 48 cases. The pattern continued over the following weeks, with 114 cases reported in HPS laboratories in week 45.
However, Dr Cowden added the worst could be yet to come with rates normally spiking during the first three months of the year. He said: "Usually the first quarter in the year will have more cases of norovirus than the last quarter of the year, but we did have a very early start to the season, so it's difficult to say what will happen.
"It could be that we had an early start to what will turn out to be a relatively normal season, or it could turn out to be a particularly bad season. I would be very surprised though if the first quarter of 2013 didn't have more cases."
The cause of the surge is unknown, but it may be greater public awareness is leading to higher detection rates or a mutation of the bug causing a surge in cases.
However, 2012 cases are still lower than 2010 when more than 3000 Scots were ill with the bug.
Residential institutions, such as care homes, were associated with 41 (82%) of the outbreaks, five (10%) were associated with hospitals, two with hotels and one with a school.
A total of 991 people were reported to be affected from the 49 outbreaks for which complete outbreak report forms were available.
Norovirus can take hold rapidly, with nausea developing within hours into severe bouts of diarrhoea and vomiting which can last for days.