11 January 2013 Last updated at 04:19 ET
A new strain of norovirus may be the reason for the sharp rise in cases of the vomiting bug this winter.
The Health Protection Agency has done tests which show there is a new variant called Sydney 2012 which has become the dominant strain in England and Wales.
It spreads quickly because people are less immune to it.
More than a million people are thought to have had norovirus since the summer, that is 63% higher than this time last year.
Dr Paul Cleary is from the Health Protection Agency.