GENEVA, Sept 25 (Reuters) - More than 2,100 people in Iraq have cholera which is spreading across the country, the World Health Organisation (WHO) said on Tuesday.

Although the number of laboratory-confirmed cholera cases has risen from 1,500 last Friday, the known death toll remains at 11, according to the United Nations health agency.

"The case fatality rate is 0.52 percent and has remained low throughout the outbreak, although it (cholera) continues to spread across Iraq and dissemination to as yet unaffected areas remains highly possible," the WHO said in its latest update.

In all, more than 30,000 people have fallen ill with acute watery diarrhoea in the past month, mainly in the northern provinces, according to figures from the Iraqi authorities. Of these, 2,116 people have tested positive for cholera.

Claire-Lise Chaignat, the WHO's global cholera coordinator, told Reuters: "There are very few deaths and we don't know why."

Chaignat said one theory for the low death rate was that fewer health workers had left the northern provinces and had been able to treat cases, and these had been mainly mild.

"The other theory is that people are seriously ill, don't have access to health care, and are dying at home," she said.

On Saturday, a case of cholera was confirmed in southern Iraq, near Basra, the first in the region, following a first case identified late last week in Baghdad.

Cholera is characterised in its most severe form by a sudden onset of acute watery diarrhoea that can cause death by severe dehydration and kidney failure within hours.

The virulent disease is mainly transmitted through contaminated water and food. About 75 percent of people infected with cholera do not develop any symptoms but the pathogens stay in their faeces for up to two weeks.

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