Hawke's Bay residents are being urged to see their doctor if they have symptoms of a stomach bug, after 45 people have been diagnosed - and one person admitted to hospital - with a "nasty" strain of the infection.
The cause of the Cryptosporidium, or Crypto, outbreak is not yet known, but health officials say it appears to be from more than once source.
Hawke's Bay District Health Board Medical Officer of Health Nicholas Jones said it was potentially spreading through day care centres, drinking untreated water, and swimming in pools, lakes and rivers.
Crypto is contracted from a parasite found in the gut of cattle and other animals and is passed to humans through contact with animals, contaminated water or food, or direct contact with human waste.The disease causes watery diarrhoea and stomach cramps, and may cause children to lose their appetite and vomit before the diarrhoea starts.
"Some people can have a very mild illness but it can be nasty," said Dr Jones. "Symptoms can come and go but have usually stopped within four weeks. In rare cases when a person's immune system is very weak the disease can go on longer and cause complications."
DHB spokeswoman Anna Kirk said one patient had been hospitalised with the infection.
"The highest rate notified so far is children under five and school age children but no common source has been found amongst them as many of them attend different schools and day care centres," she said.
Dr Jones said the Health Protection team was analysing information from all cases to establish whether there was a common source of the outbreak, but so far it looked like there was more than one source.
"We had a slightly higher number of cases than usual in spring, which is when we see infection associated with calving and lambing. Since then case numbers went back to normal and didn't increase again until after the New Year.
"Now it looks like the disease could be spreading through swimming pools, day care centres or untreated drinking water."
There was no evidence that city water supplies were causing problems.