BAGHDAD, Sept. 20  The first cases of cholera appeared in Baghdad on Thursday, in a sign the epidemic that has already sickened thousands in northern Iraq is now spreading more widely in a population made vulnerable by war to a normally preventable disease.

The World Health Organization and Iraqi Red Crescent Society reported two cases here and Iraqi television reported another case, in a 7-month-old baby, in Basra, far to the south.

People contract cholera by ingesting water or food contaminated with the feces of an infected person. Roughly one in 20 infected people become severely ill, with profuse diarrhea, vomiting and leg cramps, while others have mild or no symptoms but carry the disease.

While cholera can kill its victims in a matter or hours, it is easily controlled through basic water treatment and sanitation measures. But in a sign of how difficult that may be in Iraq, the director of the Basra health ministry, Dr. Ryadh Abdul Ameer, said Thursday that some waterworks in his city were now entirely without chlorine, which is used to purify, because imports of chlorine dried up this year after insurgents used the chemical in bomb attacks.