Remember that the symptoms of a cold or of the flu are signs that the immune system is fighting hard to combat the illness. Fever helps to speed white blood cells to their destination—to kill microbes.
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Restoring the Temple – To Arms with Your Immune System, Part I
Our powerful immune systems wipe out the invaders before they ever knew what hit them. Only when a microbe launches a particularly strong invasion (or the microbe itself is particularly virulent) does our immune system put its entire arsenal into play, stimulating symptoms such as tiredness, fever, runny nose, and coughs.