Mucus, mucins, glycoproteins, hydration, polymeric structure, mucin network, ionic interactions, pH, osmolarity, capillary forces, protein adsorption, biofilm, hyperviscosity, bronchial secretions, nasal mucus, gastric mucus, cervical mucus, intestinal mucus, ocular mucus, mucociliary clearance, pathogens, particulate transport, particle trapping, viscosity, elasticity, permeability, lubricating properties, structural heterogeneity, entangled polymers, molecular diffusion, sol-gel transition, van der Waals forces, interfacial tension, microbial adhesion, enzymatic modification, biodegradability, mucus-aerosol interactions, natural filtration, biocompatibility, oxidative stress, antimicrobial properties, physicochemical properties, mucus and inflammation, mucus and respiratory diseases, mucus and biofilm, mucus and drugs, mucus rheology.