Visitors are being warned to stay away from Tokoroa Hospital after an outbreak of the norovirus stomach bug.
Ten staff and 10 patients have the highly contagious virus, which causes vomiting and diarrhoea.
The Waikato District Health Board says the staff have been told to stay at home, while the patients have been isolated in single rooms or separated in multi-bed rooms.
The hospital's manager, Joanne Knight, says infection control measures are in place, which include strict hand hygiene.
She says an increase in norovirus in the community is common at this time of year.