Kalla-Nohra Syndrome was a terminal illness caused by a mining accident at the Gallitep labor camp. The accident affected both the Bajoran workers and their Cardassian overseers, all of whom inhaled toxicgases. Kalla-Nohra was unique to Gallitep due to the circumstances involved in the accident. Treatment was similar to that of Pottrik Syndrome.
It is not clear why the Bajoran survivors of Gallitep had their mouths covered when they gathered outside ConstableOdo's security office. As Marritza had no such covering and did not indicate any trouble breathing or sensitivity to allergens, it is likely that his exposure to the accident was less severe. The disease may also have been in its earlier stages for Marritza, or perhaps it affected Bajorans and Cardassians differently.
Likewise, the exact reasons for Kalla-Nohra being unique to Gallitep were not explained. The function of the disease for the purposes of the episode was to provide an irrefutable key to Marritza's true identity, one with which not even Kira could argue.