A highly infectious virus affecting York District Hospital has now spread to seven wards - and the outbreak could last for another week or even longer.

Hospital chiefs have stressed that the illness, which causes stomach upsets and diarrhoea, usually does not seriously affect victims.

But all routine non-emergency operations have been cancelled until after Easter, and seven wards have been closed to new admissions. The Evening Press last night reported that five wards were closed. Today York NHS Health Trust released a statement stating that 29 patients and 12 staff were currently affected by the virus - and putting the total number of people affected at 117 patients and 29 staff. Six new cases were reported overnight.

Dr Will Patterson, consultant in communicable diseases (public health) for North Yorkshire Health Authority, said: "While everything possible is being done to contain this outbreak, experience would indicate it may be another week or two before the hospital returns to full operation."

Dr Patterson said the cause of the outbreak had been confirmed as a Norwalk-type virus infection.

He said although this type of virus could be transmitted through food, it was normally transmitted person to person and is highly infectious.

"It's nothing whatsoever to do with hygiene or food in the hospital," he added.

Dr Patterson said there had been an epidemic of the virus in the UK in the last three to four years, with every hospital in the UK having a ward affected by the bug at least once a year. The symptoms tend to clear up in 24 to 48 hours.

"It's very rare that you find a fatality. It causes a short, mild, self-limiting illness," he said.

Dr Patterson said the infection may have been brought into the hospital by a patient, staff member or visitor. "A classical symptom is the sudden onset of projectile vomiting and this appears to be the main method of spread. Should the virus enter a residential setting such as a hospital, the main objective of control procedures is to contain the virus and prevent its spread to other units."

Trevor Phillips, assistant director of environmental health at City of York Council, said his department was aware of the outbreak but would not get involved unless it spread outside the hospital. George Wood, deputy chief executive of York Health Trust, said: "We thank the public for their co-operation during this difficult period and are doing everything we can to get services back to normal as quickly as possible."