North Wales is no more prone to problems with drinking water quality than other areas, says Welsh Water.
The company said the current boil water notice for 45,000 customers around Bangor, Gwynedd and parts of Anglesey was purely a precautionary measure.
It added that lessons had been learned from an outbreak of cryptosporidium in Gwynedd and Anglesey three years ago.
Anglesey MP Albert Owen has called for an inquiry. Customers are each to receive £25 compensation.
A Welsh Water spokesman said the industry had learned from the outbreak three years ago and water monitoring standards had been revised.
"We have carried out risk assessments at all Welsh reservoirs, and we now monitor our treatment works extremely closely so that we can act immediately if we see any indication of cryptosporidium.
"This is precautionary because we aim to avert an outbreak of cryptosporiodiosis in the community."
He added that a £150m programme to upgrade water treatment works in Wales is planned, and this should further reduce the risks of customers being asked to boil their drinking water in the future.
"There is no reason to think that north Wales is more vulnerable than any other area in Wales," he said.
Welsh Water issued its third warning in three years after a parasite was detected at a treatment plant.
The company said: "As we expect the boiling notice to be in place for over a week, we will pay all the customers affected £25 to cover the cost of inconvenience of having to boil their water.
"People do not have to contact Welsh Water to receive the money, it will be sent to all customers in the affected area by post over the next few weeks."
AREAS ADVISED TO BOIL DRINKING WATER
Bangor, Bangor side of Felinheli (marina), Beaumaris, Bethesda, Capel Curig, Cwm y Glo
Gaerwen , Gallt y Foel, Gerlan, Llanddaniel, Llanddeiniolen, Llandegai
Llandegfan, Llanfaes, Llanfairpwll, Llangoed, Menai Bridge, Mynydd Llandygai
Pentir, Rachub, Rhiwlas, Sling, Talybont, Tan y Foel, Tregarth
Source: Welsh Water
But Anglesey Albert Owen MP is now calling for an inquiry into the water problems.
"This is the third time in as many years that people have had to take precautions against the cryptosporidium bug," he said.
"We cannot take any risks with public health and, while I welcome the compensation to be paid to customers in the affected zone.
"What I am calling for is an inquiry into events of the last three years so that the plant at Mynydd Llandegai be given a higher priority in the investment programme to ensure that this type of incident does not happen again."
Diane McCrea, chair of the watchdog Consumer Council for Water Wales, said there was "no room for complacency" and that clean and safety water must be the priority.
She said: "I understand that the company is in the process of upgrading the treatment works which is involved in this outbreak.
"I will be calling for the company to get its other remedial works done as quickly as possible," she added.
Mrs McCrea welcomed stringent new regulations requiring water companies to give consumers warnings as soon as there was any evidence of bacteria in water supplies.
"It's no good waiting until people are ill. They have to be given the warning to boil their water and take precautions themselves."
One resident, Hanna Huws, of Llanfairpwll, who has to boil water for the second time in three years, said: "We have a grandmother living here - my mother-in-law - and she's very fragile, so I'm concerned as to how that would affect her.
"And also of course there's all the boiling of the water - there's seven of us living in the house."
The area's tap water comes from the Mynydd Llandegai water treatment works, which takes supplies from the Marchlyn Bach and Ffynnon Lligwy reservoirs.
Cryptosporidium is a parasite found in humans and animals and can cause temporary diarrhoeal illness. Its eggs are killed by boiling water.
The National Public Health Service for Wales has asked local GPs to report any cases of diarrhoea and to take samples to be tested for gastrointestinal infections such as cryptosporidium.
It is "closely monitoring" the levels of illness in the area affected and advising people to maintain good personal hygiene.
For several days in mid-August, almost 5,000 people in the Tywyn, Aberdyfi, Bryncrug, Rhyd-yr-Onen and Brynglas areas of Gwynedd were also advised to boil their tap water after the quality was affected by heavy rainfall.
In November 2005 a total of 231 people were left ill in Gwynedd and Anglesey after cryptosporidium affected a Snowdonia reservoir.
The water company was fined £50,000 after admitting supplying unfit water in that case.
An investigation is under way by the Drinking Water Inspectorate into the latest case and Mrs McCrea said she would be urging it to make its report public as quickly as possible.
She will also ask the water company to distribute bottled water immediately to people on their vulnerable register.
Any customers who want more information can call 0800 052 0130.