I have the flu. Why do I have a headache?

Firstly, because your body is full of inflammatory chemicals and this makes more or less everything hurt.

Secondly, because catarrh filling the sinuses in your head creates pressure and thus causes pain.

Thirdly, because you are likely to have sweated out more water than you’ve taken in and become mildly dehydrated.

Fourthly, if you have lost your taste for coffee, tea or cola drinks whilst feeling so poorly, you will be experiencing mild withdrawal symptoms, which commonly include a headache.