Migraine Headaches.

In the western world about ~18% of women and ~6% of men get migraines. I'm one of those 'lucky' guys who get them, not all that often thankfully, but they are more likely to occur when there's some stress around.

Christmas day I had a doozy. So much so that I didn't go to Christmas lunch with my family.
I'm told that they think it's caused by some blood vessels in the head dilating, and stretching the nerves near them, which causes the pain, but there are many other theories as well.

Hard to explain a migraine to someone who's never had one, but it can be pain so bad it sends you into shock, certainly enough usually to cause actual sickness, and often effects on your vision too.

I hope you don't get them, but spare a thought for those in your circle who do.