In the past two months more than 80 cases of the stomach bug cryptosporidium have been reported in the lower South Island.
The Southern District Health Board is calling for vigilance and the board's medical officer of health Marion Poore said the cases were twice the number reported in Otago and Southland than is usual at this time of year.
Children under five have been hit particularly hard with 25 preschoolers affected.
The parasite lives in the gut of humans and animals, and can cause diarrhoea, nausea and stomach pain.
Dr Poore said the best way to avoid the disease was good hygiene with hand washing particularly important.
She said those infected should avoid work and school until symptom free for 48 hours.