As the body fights against the virus, the temperature increases. This results in a vasodilatation of blood vessels throughout the body, including the brain, which results in a headache. Another reason may be sinusitis that often develops during the influenza infection (particularly in people who already suffer occasional episodes of sinusitis).
In the absence of sinusitis, as the body temperature drops, the headache decreases. A persistent headache that intensifies when coughing is a characteristic of sinusitis. Rarely, some patients report a severe headache accompanied by a stiff and painful neck. This symptom requires immediate medical attention.