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The Age UK Imagine if WHO declared a pandemic for a novel strain of the common cold, with symptoms no worse than typical colds give, and the governments closed every school in their countries, closed the borders, and sent armed marines to deliver medicine ...
Mucus turns yellow or green whenever you have a bad respiratory infection, viral or bacterial. The colour comes from myeloperoxidase, an enzyme in white blood cells which uses an iron-containing haem molecule, which is green, to catalyse chemical reactions that kill germs.