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Updated at 6:26 pm on 16 January 2013
The Ministry of Health says a new mutation of the stomach bug Norovirus has already caused a rise in the number of cases in New Zealand.
However, it is disputing statements by Australian researchers that more than 100,000 people in New Zealand are likely to be infected.
The virus is thought to be a hybrid of strains from the Netherlands and Japan and was identified by a team at the University of New South Wales in March last year.
The researchers say while the Sydney 2012 variant is no more contagious than other forms of Norovirus, it is not recognised by people's immune systems.
Professor Peter White told Radio New Zealand's Summer Report programme on Wednesday the virus is highly contagious and those with it must keep away from work, hospitals, young children, the elderly and anyone with compromised immunity.
Professor White said it will be very difficult to avoid the virus, which causes violent vomiting and diarrhoea.
Fran McGrath, the ministry's acting director of public health, says the variant has been seen in New Zealand since October last year.
Dr McGrath says that pushed the number of outbreaks in groups such as schools or resthomes to 226 last year, 45 more than in 2011.
The number of individual cases in 2012 was 5487 - far from the number predicted by the researchers.
The Aged Care Association is warning members of the public against visiting resthomes and hospitals if there is a chance they could be infected with Norovirus.
Chief executive Martin Taylor said hand hygiene is critically important when it comes to protection.
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