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In page Cystic fibrosis:

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As water follows sodium, the depth of ASL will be depleted and the cilia will be left in the mucous layer.[12] As cilia cannot effectively move in a thick, viscous environment, mucociliary clearance is deficient and a buildup of mucus occurs, clogging small airways.[13] The accumulation of more viscous, nutrient-rich mucus in the lungs allows bacteria to hide from the body's immune system, causing repeated respiratory infections.[citation needed]