One who gains admittance, as to a party or concert, without being invited or without paying.
Underastanding migraine first requires a change of perspective because it's still seen as being 1. A headache and 2. Episodic (ie it comes and goes).
Well migraine is not a headache and because it's genetically linked it never 'goes away'. What's more over time the symptom of headache can become secondary to the other more diffuse and challenging aspects of migraine itself.
One big function of the brain is to STOP feelings or sensations. It's called INHIBITION. But the disorder of migraine results in a brain which has trouble stopping normal sensations (from the head or guts or anywhere else) getting through.
As a result a migraine sufferer typically gets head, gut, leg, body discomfort/sensitivity and is prone to extreme fatigue, anxiety/depression and is more likely to suffer from a whole host of so called co-morbid conditions many of which are likely manifestations of the disease itself.