The largest percentage of headaches (90%) are primary (idiopathic) ie. without underlying cause. These include tension headache, migraine with or without aura, cluster headache, chronic use of analgesics, and cervical headache..
On the other hand, secondary headaches may be due to a number of diseases such as infections (meningitis, encephalitis), tumors (primary or metastatic), subarachnoid hemorrhage, cranial nervalgia and ophthalmic neuralgia.