The US and the UK are responsible for this.
The World Health Organisation was warning of a possible epidemic immediately after the war because water and sewage treatment facilities were deliberately destroyed in the 'shock and awe' bombardment leading up to the invasion. 
These facilities were never properly repaired since then despite clear legal duties imposed on the occupying forces to restore basic services and security. 
Cholera is easily prevented by chlorination of the water supply. However since chlorine has been used in several bombs this year, it has been denied to water-treatment plants. Currently 100,000 tons of chlorine has been prevented from entering the country from Jordan for fear that the occupying forces couldn't safeguard it.  Cholera is killing more people than the chlorine ever could but the occupiers prefer 'natural deaths' to possible combat deaths.
"Iraqis commonly use chlorine for purifying water and as a general disinfectant. Qasem al Mussawi, the spokesperson of “Operation Imposing Law” said that water treatment stations are the main source of chlorine for the terrorists. Insurgents have raided nearby water purification plants to steal the chemical. Iraqi General al-Musawi stated that chlorine from water purification plants was being smuggled to the armed groups. The Arabic news wire service, Almoheet, reported that the government is also considering drastic limitations on chlorine use and possibly a complete ban of the chemical substance." 
There is another possible explanation for the unprecedented speed of the epidemic - deliberate biological warfare by the occupying forces. While it may seem outlandish to suggest such illegal use of WMD by US and UK forces, it is not unprecedented. US forces used cholera and plague against Chinese and North Korean forces during the Korean war.
"I went to China in 1952 wanting to assess the assertions of germ warfare being one reason. Without going into the evidence, I came away convinced that Chinese officials believed that the evidence was conclusive. On returning, Alan Watt, my successor as permanent head of the Australian Department of External Affairs, informed me that in the light of my public statements he had sought a response from Washington and was informed that the United States had used biological weapons during the Korean war but only for experimental purposes" - Dr. John Burton, letter of 12 April 1997 
 http://www.nytimes.com/books/first/e/endicott-biological.html or