I share Kerry's take on this. The majority of hangover symptoms, including headache, are caused by dehydration. Keep in mind that dehydration often happens without/before any feeling of thirst.
As for a possible viral cause, my vote would be
gastroenteritis a.k.a. 'stomach flu'. Either way, the 'cure' is the same: rest, foods -- and over-the-counter pain reliever (such as Tylenol) as needed.
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Steve
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