Strollerderby

One medical condition that might result in a person being "scared to death" would be long QT syndrome.  It's a heart rhythm disorder, often genetic, that can result in a person's heart spontaneously losing its rhythm and in the worst case leading to the person's sudden death.  According to the Mayo Clinic, this can happen when the person is "excited, startled, scared, or during exercise".  So if someone has this condition (which might not be obvious, sudden death can be the first "symptom") you might indeed be able to scare them to death.

Breath holding spells can make a child look as though they are dying, although they are actually harmless to a child. In response to extreme emotion, like pain, anger or fear, a child can cry so hard that they stop breathing and lose consciousness. My son has had two of them, thankfully he hasn't had one in over 6 months, and they are terrifying. The first time he had one, I actually thought that he was dying. His whole body turned purple and his eyes rolled up in his head. Thankfully, a child can't actually die from one. But, watching one might just scare a parent to death.

Holy crap Diera, you are the first person i've ever known that wasn't in my family that knows about long qt syndrome. My sister and 2 of my cousin's have it, and my aunt died from it at a young age. I don't dare try and scare any of my neices or nephews for fear of this, and I hate it when anyone does that to them. I remember watching something a few years ago (like Oprah one of those shows) where a mother was playfully scaring her little girl and she literally was "scared to death"...Of course I don't remember what caused it or anything like that, but sounds like it can indeed happen.

Sara,

I'm sorry to hear about your aunt and it must be scary to worry about your family like that.  The reason I know about it (besides the fact that I tend to collect medical trivia) is because a woman on another kid-related site I used to follow had her husband die suddenly of it when their child was an infant.  He had a cold, and she went out to get him some cold medicine, leaving him with the baby, and when she got back he was gone.  She was incredibly brave about it, but it was such a scary story that it's stuck in my head even though it was years and years ago.  

Thank you, luckily (I think) I didn't know her and it happened waaaay before my time. But yes it still is incredibly scary, my sister & my cousins all have pacemakers & defibrillators to control it, but even when they have episodes those are scary enough as it is. I do know that my sister said there are like two different types of it, one being fatal, the other not. Of course she would have the fatal type! That's awful about what happened to that poor woman. I can't imagine what that would be like. So so sad.