On Tuesday, the Eastern Health Board issued a statement stressing there was no link with last year's outbreak.
The cryptosporidium parasite can cause vomiting and chronic diarrhoea, and is particularly debilitating for the elderly, young children and the sick.
The Eastern Health Board, which serves the eastern part of the province, has recorded 87 cases so far this year.
The board said that well over half of the cases had occurred since the start of April.
In Belfast there have been more than 64 cases since the end of February, a rise of 14 from earlier this week.
Most have been in the south and west of the city but it is not known what else links them.
There has been a call for Northern Ireland Health Minister Bairbre de Brun to take action to control the outbreak.
South Belfast SDLP assembly member and GP Alasdair McDonnell said it was important the source of the disease was found.
On Friday, Dr McDonnell warned that people could die unless the disease was quickly brought under control.
"It has to be got on top of very quickly," he said.
Urgency
Regional Development Minister Gregory Campbell has said the Water Service was looking at its treatment processes and distribution system as a matter of urgency.
However, he assured the public that water was safe to use and drink.
Ciaran Boyle from Belfast is among the people who took ill last week.
He has Crohn's disease and the drug he takes for it affects his immune system.
He has been interviewed by public health officials who are trying to establish anything which he might have in common with other sufferers.
He said: "I have no idea where I picked it up because I have just been drinking the water here at home."
About 100,000 people were affected by an outbreak of the bug last August.
Infected water supplies at Twinbrook and Poleglass, on the outskirts of Belfast, were blamed.
On Tuesday, the Eastern Health Board issued a statement stressing there was no link with last year's outbreak.
It said: "There is an expected seasonal increase in cases of cryptosporidium at this time of the year.
"However, in the current year the increase is greater than usual.
"The board therefore recommends good hygiene practice - especially hand-washing before eating and avoidance of handling or preparing food for others when ill."
It warned that people with severely impaired immunity, such as Aids sufferers or people receiving chemotherapy, should boil all water, including bottled water, before consumption.