Cough: Coughing is a body’s way of clearing irritants (like allergens, mucus or smoke) from airways and preventing infection. Coughs may be dry or productive. A productive cough (wet cough) brings up sputum (phlegm, mucus and other matter) from the lungs. Cough is commonly caused as a response to allergy or viral infection but coughing up mucus is possibly an indication of infection in the respiratory tract. If you are coughing thick, solid white mucus, it indicates you may have a bacterial infection in the airways. If the mucus is clear or light-coloured, it indicates you may have allergies or minor infection in the respiratory tract and yellow or green mucus indicates a respiratory infection.