When you have appropriately hydrated, the mucous membranes of the nose becomes more moist and prisoners virus before they do harm. It is especially important in dry air, which tends to dry up the mucous. Tea, primarily the green, is also a good choice because tea contains immune substances tonic-type polyphenols.
Follow your thirst. Your body needs some. 2.5 liters of water a day. One liter you get from your food, so in practice you should drink 1.5 liters of water daily to maintain hydration. Obviously something more if you sweat a lot or it is very hot. Very large quantities of water serves no purpose, because you risk flushing useful vitamins and minerals in your body.
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