Tens of thousands of households across north-west Wales could have to boil their water beyond the new year, public health officials have warned.
The Cwellyn reservoir in Gwynedd is seen as the most likely source of the stomach bug, cryptosporidium.
Experts say the outbreak should burn itself out, but if it does not show signs of that by early January, the warning to boil water will be extended.
So far, 110 people have been taken ill in Gwynedd and Anglesey.
Ysbyty Gwynedd, Bangor and Eryri Hospital in Caernarfon have confirmed patients will be given only bottled water, and signs will be put up warning people not to drink tap water.
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
The National Public Health Service for Wales said the source of the cryptosporidium infection had not been established, but investigations were centred on the reservoir, which serves 70,000 households.
Dr Mark Walker said: "Unlike other water-borne outbreaks of cryptosporidiosis we can't find anything in the water, or any event that has occurred in the water treatment that could account for this.
"Cwellyn seems to be the most likely source. What we don't know is how contamination, if it got into Cwellyn, got into there.
"So we're engaged in a detailed environmental investigation, looking at how water gets into Cwellyn, how it flows, from the water coming in, to the water going out, and so on."
He said that the bug still accounted for only a minority of cases - around 10% - of diarrhoea and vomiting cases in the area.
He added that the advice to boil water would be renewed if, by 9 January next year, the outbreak was continuing.
"If there's evidence the outbreak is ongoing, we will extend the boiling water notice but experience from the past suggests that water-borne outbreaks do burn themselves out with time."
Dwr Cymru is writing to 70,000 customers in the area which obtain water from the reservoir, at Rhyd Ddu.
The bug causes unpleasant stomach problems for most people although those with immune system deficiencies could suffer longer term problems.
Dr Phil White, a GP with surgeries Menai Bridge on Anglesey and on the mainland, said the bug would pass through a person's system in time. He urged people to carry out stringent hygiene measures such as hand washing.
He said: "Boiling water is a very sensible step. If it has been identified as the source of the infection then this is a very sensible precaution.
"The cysts are destroyed by just boiling the kettle once, you don't need to continuously boil it, just bring it to boiling point."
Beth Angell, in Caernarfon, said her two daughters and her mother had all been ill with stomach problems.
She said: "It's concerning, but the biggest worry is the lack of information because the minute you hear 'bug' everyone starts to panic, especially if it's in the water supply, it's like the stuff that Hollywood films are made of.
"When you go on holiday you're told don't drink the ice, don't wash the fruit, don't clean your teeth in the water in the taps, and the only information I've seen was on the BBC news last night, and I haven't heard anything further than that."