People have been told to boil their drinking water after 22 cases of a waterborne disease were confirmed in and around a town in South West England.
"Small traces" of a parasite which can cause a disease with diarrhoea symptoms were found in the water supply in Devon on Wednesday, according to South West Water.
It added it is working with the UK Health Security Agency (UKHSA) to "eliminate the source" of the disease and that bottled water stations will be set up in the areas affected. One has already been set up to distribute bottled water at the Broadsands Beach car park in Paignton.
The detected parasite - cryptosporidium - can cause cryptosporidiosis disease when it is passed on to humans.
Anyone can get the disease and it can be a serious illness in people who have immune systems that are not working properly, according to the UKHSA - though most healthy people should recover.
Infections can be caused by drinking contaminated water or swallowing contaminated water in swimming pools or streams. It can also be acquired by animal and human contact.
South West Water said: "Customers in Alston and the Hillhead area of Brixham are advised to boil their drinking water before consuming following new test results for cryptosporidium.
"We are issuing this notice following small traces of the organism identified overnight and this morning."
As well as the 22 people confirmed to have the disease, as many as 70 other reported cases of diarrhoea and vomiting in residents and visitors to Brixham are also under investigation and more confirmed cases are expected.
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What is the parasite causing disease outbreak?
Thomas Moore
Science correspondent
Cryptosporidium is a microscopic parasite that commonly lives in the guts of wild and domesticated animals, and can contaminate reservoirs and rivers that water companies use as a source for their supply.
It can cause really unpleasant diarrhoea and vomiting for several weeks.
Children under the age of 5 and people with weak immune systems are likely to be worst affected, with dehydration and weight loss. In severe cases people may need hospital care.
There is no specific treatment for the parasite itself. But people who are infected need to drink plenty of fluids. It's also a good idea to talk to a pharmacist about oral rehydration sachets to help replace the sugar, salts and minerals the body has lost.
The illness, called cryptosporidiosis, is highly infectious and is easily passed around a household. The UK Health Security Agency says hygiene is important. It recommends handwashing with soap after using the toilet or changing nappies, and before eating. It also advises washing bedding and towels on the hottest possible cycle.
People shouldn't prepare food for others until they have been symptom-free for at least 48 hours. The Agency also urges people not to return to work or school until two days after symptoms stop.
The disease is so unpleasant that alarm bells ring when the parasite is found in the water supply.
It's a challenge to kill because its small size means it is difficult to filter out. It's also resistant to chlorine, which kills most other bugs to give us clean tap water.
That's why water companies have to assess the risk of the bug in the source of their supply. It's a regulatory requirement and failure to comply is an offence.
South West Water added it was "urgently investigating" and apologised for the inconvenience.
Sarah Bird, consultant in health protection at UKHSA South West, added: "We advise people in the affected areas to follow the advice from South West Water and boil their drinking water and allow to cool before use.
"Anyone with a diarrhoeal illness should drink plenty of water to prevent dehydration and if they have severe symptoms like bloody diarrhoea, they should contact NHS 111 or their GP surgery."
Cryptosporidiosis symptoms include watery diarrhoea, stomach pains, dehydration, weight loss and fever, which can last for two to three weeks, the UKHSA added.
Ms Bird continued: "For most people, cryptosporidium symptoms can be managed at home without needing medical advice.
"Those affected should stay off school and work for 48 hours since the last episode of illness and away from swimming pools for 14 days after the last episode of illness."
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Torbay Council had said on Tuesday that South West Water had been carrying out tests for the parasite.
After initially finding the water supply was safe and fine to use as normal, the water company issued an update with the tap water warning on Wednesday.
It comes after hundreds of residents said on the Facebook group Brixham Fish Town that they have fallen ill in recent days.
A post to the group on Tuesday had more than 1,200 comments from locals saying they or a family member were sick.
One person said on the group they were "fuming that this could happen" after confirmation of the parasite in the water supply.
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Anthony Mangnall, the Conservative MP for Totnes, said he is "aware of current reports of illness in Brixham, and concerns regarding the quality of drinking water".
He added he knew of "many more reported cases of diarrhoea and vomiting in residents" in the town.
The cause of the outbreak has not yet been confirmed.
