"There were more cases of shigellosis this year than there would [normally] be in a month.
"It's hard to explain why there's an increase."
He said while three of the cases could be explained as a household cluster, no links had been found between the other cases.
"We initially thought it was some kind of outbreak ... but it's scattered around the Wellington region.
"The other four people appear not to be linked together, we are not entirely sure why these four have turned up.
"We have investigated pretty thoroughly, we have looked very closely at the DNA fingerprints of the bug and they all seem to be different."
"Often we find shigellosis is linked to overseas travel but none of them had travelled."
Most people who get it have either travelled overseas recently, particularly in India, Nepal, South East Asia, and the Pacific Islands, or have had contact with someone who has recently travelled overseas.
Shigella bacteria is frequently found in water contaminated by human sewage.
Dr Thornley said it can cause nasty diarrhoea.
"It's more severe than the average salmonella."
Shigellosis is caused by the shigella bacteria that live in people's intestines.
Typically, an infected person's faeces contain blood and mucus resulting from the invading bacteria causing small ulcers and abscesses in the wall of the intestines.
A person is usually fully recovered within two weeks, but young children and the elderly can become very ill with dehydration and can be hospitalised for a short time.
Dr Thornley said the disease can be transmitted in food, from infected people contaminating it, but close, personal contact was also a risk factor.
He said the main way to prevent it was to continue classic hand hygiene techniques and be cautious.
Those in a household with someone with diarrhoea should follow the 20/20 rule of 20 seconds washing with soap, rub and rinse with running water and 20 seconds drying with a clean towel, he said.
"That should usually continue until two days after the person's symptoms have settled down."
