During the first 44 weeks of 2012, there were 2016 laboratory reports of norovirus received by Health Protection Scotland (HPS), a rise of 631 (46%) compared to the 1385 received in the same period last year. HPS says 131 of the cases occurred during two weeks in October. In week 43 there were 48 reports rising to 83 the following week.
It is estimated that for every laboratory case there is about 290 more in the community, with an estimated 180,000 additional cases of the bug so far this year.
Chief medical officer Sir Harry Burns said: "Rates of norovirus fluctuate year to year with occasional spikes, so we cannot estimate how severe this winter season will be. We remain vigilant and ready to cope."
Mr Burns last month urged health professionals and patients to look out for signs of the illness, which can be fatal in people who are already very weak or ill.
He said cases of the vomiting bug had started earlier than usual this year and added: "This may be indicative of a higher than normal level of norovirus circulating this winter."
He assured patients that health boards were "ready to cope with whatever the winter has in store".
On Tuesday NHS Highland announced a ward at Raigmore Hospital, Inverness, had closed following an outbreak of norovirus. Yesterday the board said the number of cases had increased. Five cases of the bug were initially confirmed by the health authority but that number has now risen to six.
The ward was closed to new admissions and visitors were asked to stay away.
Last winter norovirus outbreaks forced the closure of 374 hospital wards across Scotland – with a peak of 22 in the worst week. But this was far fewer than in winter 2009-10 when 757 wards were shut.
Health authorities reported at least 116 separate norovirus outbreaks in the nine months to September last year. Most were confined to care home and hospital settings, although there were high-profile outbreaks in schools, restaurants, cruise ships and hotels.
An outbreak at Monklands Hospital in Airdrie, North Lanarkshire, shortly before Christmas last year, resulted nearly 100 staff and patients being struck down, with wards closed and operations cancelled.
The Scottish Government's advice to those who catch the winter vomiting bug is to drink plenty of non-alcoholic, non-milky liquids to replace lost fluids.
Infected people should make sure their hands are clean to prevent the illness spreading and avoid preparing food for other people.