Why norovirus is highly infectious
15 December 2012 Last updated at 20:27 GMT
Hospitals around the UK have been closing wards to visitors in the hope of preventing the spread of the winter vomiting bug, norovirus.
Birmingham's City Hospital, Maidstone Hospital in Kent, and George Eliot Hospital in Nuneaton, Warwickshire, all have patients with the infection.
At Southampton more than 400 virus-infected passengers disembarked from a cruise ship.
Virologist Dr Chris Smith, who has succumbed to norovirus a number of times himself, explains why it is so infectious.