The nasal sinuses and passages are such a unique set up.  They are essentially air containing spaces lined with mucous membranes designed to warm, filter, and humidify the incoming air for our lungs.    The mucous membranes have cilia on them to move the secretions back towards the throat.  There are bacteria and fungi throughout the nasal cavity and sinuses termed normal flora, so the difference between infection and colonization is really detrimental symptoms.