Sudden death syndrome - when I started on BYC 6 years ago I heard about that often(in many breeds) except they just called it "flip," Just suddenly a bird on it's back, dead. It was believed to be caused by a heart attack. I don't know if any research was done to validate that
I have heard that Seramas can carry a lethal gene that can kill them in the egg or shortly after. I'm not an authority on any of this. I just did some research years ago when my friend wanted to buy a couple serama hens. Knock on wood, her seramas have been the most robust breed she has had in years.