I have/had a chronic tension type headache/migraine complex that held on for several years (it's still abating).

Have you ever had mono, or other exposure to the EB virus? Once you've had it, it usually lays dormant for the rest of your life, but in some people it can be reactivated by another virus (like your flu—that's how mine began) and cause chronic headache/migraine complexes.

A typical neuro isn't likely to know/have heard of this; I bounced around between a half dozen or so neuros until I found one whose practice was solely headaches/migraine.

Doc