A nasty bug, causing vomiting and diarrhoea, is affecting people in Nelson and Richmond.
GP spokesman Graham Loveridge said that it was most likely a viral infection, similar to norovirus or rotavirus.
"It mostly causes vomiting and diarrhoea and lasts anywhere from 24 to 72 hours," he said.
With schools and kindergartens going back this week it was important parents were aware of it.
Children, and adults, should stay at home for at least 24 hours after they experienced the symptoms, he said.
"It's very contagious. There's a risk of dehydration, particularly in little kids and older folk."
It was important to make sure fluids were kept up, even if it was only a tablespoon every five to 10 minutes, he said.
"People should also take care with handwashing. If parents are looking after a sick child, soap and water is their best friend."
Alcohol-based handwashes and sanitisers were also effective.
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