<P> In the case of a viral infection such as cold or flu, the first stage and also the last stage of the infection cause the production of a clear, thin mucus in the nose or back of the throat . As the body begins to react to the virus (generally one to three days), mucus thickens and may turn yellow or green . Viral infections cannot be treated with antibiotics, and are a major avenue for their misuse . Treatment is generally symptom - based; often it is sufficient to allow the immune system to fight off the virus over time . </P> <P> Increased mucus production in the upper respiratory tract is a symptom of many common ailments, such as the common cold . Nasal mucus may be removed by blowing the nose or by using nasal irrigation . Excess nasal mucus, as with a cold or allergies, due to vascular engorgement associated with vasodilation and increased capillary permeability caused by histamines, may be treated cautiously with decongestant medications . Thickening of mucus as a "rebound" effect following overuse of decongestants may produce nasal or sinus drainage problems and circumstances that promote infection . </P> <P> During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined . As a result, the nasal cavity can fill up with mucus . At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils . This causes an excess amount of water to build up inside nasal cavities . In these cases, the excess fluid usually spills out externally through the nostrils . </P> <P> Excess mucus production in the bronchi and bronchioles, as may occur in asthma, bronchitis or influenza, results from chronic airway inflammation, and hence may be treated with anti-inflammatory medications . Impaired mucociliary clearance due to conditions such as primary ciliary dyskinesia may also result in its accumulation in the bronchi . The dysregulation of mucus homesotasis is the fundamental characteristic of cystic fibrosis, an inherited disease caused by mutations in the CFTR gene, which encodes a chloride channel . This defect leads to the altered electrolyte composition of mucus, which triggers its hyperabsorption and dehydration . Such low - volume, viscous, acidic mucus has a reduced antimicrobial function, which facilitates bacterial colonisation . The thinning of the mucus layer ultimately affects the PCL, which becomes dehydrated, compromising ciliary function and impairing mucociliary clearance . </P>

Where is mucus produced in the human body