Public health officials say 110 people in north Wales have now contracted a water-borne stomach bug.
Some 70,000 households in Gwynedd and Anglesey will have to boil water into the new year while the source of the bug, cryptosporidium, is traced.
However, deputy chief medical officer Mike Simmons, said he was much less worried about this outbreak than the "massive" E.coli one in south Wales.
People have been told to boil water or buy bottled water until 9 January.
The source of the cryptosporidium infection has still not been traced although the Cwellyn reservoir at Rhyd-ddu in Gwynedd has been constantly monitored since the outbreak was first confirmed a week ago.
BOIL WATER NOTICE AREAS
Bangor, Beaumaris, Beddgelert
Bethania, Bethel (Gwynedd), Betws Garmon
Bontnewydd, Caeathro, Caernarfon
Carmel (Gwynedd), Ceint, Cwm y Glo, Dinas
Ffridd Uchaf, Gaerwen, Groeslon
Llanberis, Llanddaniel, Llandegfan, Llandwrog
Llandygai, Llanfaes, Llanfaglan
Llanfairpwll, Llangoed, Llanrug
Llanwnda, Menai Bridge, Nantmor
Penmynydd, Penyffridd, Plas Gwynant, Pont Aberglaslyn
Pont-rug, Rhosgadfan, Rhostryfan
Rhyd-ddu, Seion, Vaynol Hall
Waunfawr, Y Felinheli
Public health officials announced 72 people had caught the infection since the beginning of October. That figure has risen by almost 40 in a week and experts predict it will rise still further.
Mr Simmons told BBC Wales: "I am less worried about this organism [cryptosporidium] that I was with E.coli but clearly I'm concerned as anyone else to try to get to the bottom of this outbreak."
During the outbreak of E.coli 0157 which emerged in the south Wales valleys in September, more than 170 people, mainly children, became ill and one boy died.
Symptoms of the cryptosporidium bug are diarrhoea and stomach cramps and it typically lasts for around a fortnight.
Water company Dwr Cymru is sending letters out to 70,000 households in the area which receive their water from Cwellyn reservoir.
Despite monitoring, no trace of the bug has been found at the reservoir
In an advert taken out in the Daily Post newspaper on Thursday, the firm's managing director, Nigel Annett, said no trace of the bug had yet been found at the site.
A Dwr Cymru spokesman added it would not have been possible to switch the supply so people received their water from another reservoir.
He said: "Because this is a difficult geographical area, we cannot pipe in water from other areas. If we tankered in water, that would have to be boiled anyway.
"What we are doing we are contacting all our customers on our special needs register, ensuring they get bottled water."
Wards and clinics
Delyth Jones, owner-manager of a nursing home in Llanrug, said she had been issued with no advice on what to do before she took the decision last week to boil all the drinking water at the premises.
She said: "We do encourage the elderly to drink. It's important there is enough water.
"We have to reassure them and it's hard to explain to them why they can't drink the tap water, they don't seem to understand."
Public health experts say the outbreak should "burn itself out", but if that has not happed by early January, the advice to boil water will be extended.
Ysbyty Gwynedd in Bangor, one of two hospitals in the outbreak area, responded immediately the warning was given on Wednesday, importing supplies of bottled water for its wards and clinics.
Professor Hugh Pennington, the expert heading the official inquiry into the south Wales E.coli outbreak, said the source of the cryptosporidium outbreak may never be found.
He said: "Boiling water into next year sounds pretty drastic, but it's in the public interest" .
Meanwhile, the National Public Health Service for Wales has responded to criticism from members of the public about difficulties getting through to their helpline.
A spokesman said the helpline would now be open an hour longer on weekdays from 0900 to 1800 GMT, and more people would be answering calls.
The helpline number for people with questions or concerns is 0845 850 9850.