I've got
Ankylosing Spondylitis - basically an autoimmune situation. At the time I was eating a lot of eggs I guess I didn't pay enough attention to my gut health, and some undigested protein made it's way into my blood stream. This sensitised my immune system to what it then sees as a foreign body invasion.
In normal folk who get this sensitivities, you get all manner of different reactions (headaches, water retention, joint pain, etc) to a greater or lesser extent. For me it turns an already over-active inflammation pathway up, and my joints get attacked by my body.
The other week I bought some sushi from a supermarket to eat on the way around (EVERY sandwich contained mayonnaise), and ate it without a thought. Only after when I looked at the packet it said contains eggs, brain into gear, there was a tiny bit of tuna mayo in one of the pieces

Next morning I was in a right load of pain!!