The return to work this week will prove "heavy going" for the NHS with many people laid low by the spread of the winter vomiting virus and flu, a health campaigner said today.
Businesses and schools will also be affected as people call in sick.
And those employees who do struggle in to work are more likely to spread bugs around offices and factories.
Geoff Martin, from campaign group Health Emergency, said: " "The period after Christmas and New Year is always heavy going for hospitals as the country gets back into gear but the return of the icy weather, along with the flu and the norovirus, is likely to pile on the pressure.
"We know from front-line reports that services in many areas are dangerously short of capacity and are working on a knife edge.
"In the north west, the service is paying a heavy price for the closure of Burnley's A&E department and bed reductions elsewhere have left the NHS dangerously exposed this winter."
Stephen Alambritis, chief spokesman of the Federation of Small Businesses, said anecdotal evidence pointed to people staying away from work - causing more problems for businesses in what are already difficult times.
He said: "Large companies can move staff from one division to another branch, but if you are a small firm with 10 people and two call in sick that is 20% of your workforce.
"Absence from work costs the UK economy £13 billion a year and this could add to it at the worst possible time."