Chronic sinusitis is treated by the application of cold plasma or plasma-activated species to the infected mucosal surfaces through use of an endoscope having a steerable end which may be projected into the sinus cavities through the nasal cavity. The cold plasma is generated at either the distal end of the endoscope with a power source by application of a power, or at the distal end by gas and electrical connections extending through the endoscope. The cold plasma or plasma-activated species act to destroy bacterial cells but not eukaryotic cells.