A headache is not a reliable indicator of the flu because it also occurs in the case of a cold, though severity can be a clue to which condition is the cause -- headaches caused by a cold are often more minor. This is also true for other common symptoms of colds and flu, with the flu causing more intense symptoms. Mucous membranes lining the nasal and sinuses cavities can become inflamed, causing sinus headaches, and fever or dehydration can also cause them.

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