Virus expert Dr Ken Flint said potentially lethal bacteria like e.coli and salmonella would be left behind in the sludge in houses and streets for weeks or even months after the floods recede.
And he claimed the elderly, the very young and the infirm were at serious risk from gastroenteritis. Microbiologist Dr Flint added: "I expect to see three to four times the normal rate for these diseases in coming weeks. That would mean the potential for low hundreds of people dying."
His stark warning came as scientists found evidence of around a dozen dangerous viruses in flood water samples taken from swamped Gloucestershire streets.
They included salmonella, the hospital superbug clostridium difficile and the virus that is commonly known as the winter vomiting bug.
Scientists say the microbes are breeding in water which has become contaminated with human waste flushed from swamped sewers.
The terrifying revelation came as Oxford became the latest flood-crisis area - after the Thames breached its banks and swamped parts of the city.
Warwick University's Dr Flint said the worrying health implications meant it was vital the hundreds of thousands of flood victims maintained a strict hygiene regime.
He added: "There is going to be a lot of raw sewage in the water.
"The worst time is going to be when the water recedes. The bacteria will be far more concentrated in the sludge.
"People will need to use the highest hygiene precautions to avoid sickness.
"Nobody in their right minds would normally touch raw sewage - but I have seen pictures on TV of children frolicking in the water.
"That is mad. If bacteria gets on their hands and they can't wash it can transfer to food."
Floods expert Professor Ian Cluckie said towns were at greater risk than rural areas as the level of sewage in the water was likely to be higher.
He added: "People need to realise this is raw sewage they are walking around in. I've seen pictures of kids walking around in the flood water. For God's sake don't let them."
Prof Cluckie said people must also take great care not to let cuts or broken skin come into contact with water.
Yesterday Shona Arora, director of public health in flood-ravaged Gloucestershire, admitted sanitation was becoming "an increasing issue".
Among those hit by flooding in the county are 25,000 over-85s, a sector of the community particularly vulnerable to killer bugs such as salmonella.
Scientists who tested the flood waters found traces of typhoid and cholera - but not at danger levels. They also identified clostridium difficile, cryptosporidium, and campylobacter - which can cause brain damage.
And there were traces of escherichia, responsible for severe diarrhoea and dysentery and the similarly potent rotavirus, which kills over 600,000 children a year in developing countries.
The Health Protection Agency yesterday said anyone who develops diarrhoea, fever or abdominal pain after being in contact with contaminated water should seek immediate medical help.
A spokeswoman added: "There has been a degree of faecal contamination. This would not be unexpected in flood water. There is a risk of pathogens which can carry gastro-intestinal infections being present in the water.
"The most contaminated water is likely to have been present in the initial floods.
"Since then it is likely that as water continues to flow from the hills that the contamination has become diluted. However we cannot rule out further contamination."
A spokeswoman for Gloucestershire Primary Care Trust said yesterday the emergency water taken from bowsers around the county is of drinking quality. But to be on the safe side it should, when possible, still be boiled first before drinking it - or using it to clean teeth. Microbiologist Dr Roderick Gibson also warned mums not to bath babies in water if it is not fit to drink.
He said: "Babies are more vulnerable to sickness and dehydrate quicker. It's better to be safe than sorry."
Around 340,000 people in Gloucestershire face being without water for up to two weeks.
The crisis began on Friday when a vital water treatment plant was contaminated by the floods.
Güncelleme Tarihi: 26 Temmuz 2007, 15:51