Feeling of disgust results in nausia

[Edward]

I was reading a book "Throway Daughter" and it had a description of someone watching footage of the Bejing massacre that person got a feeling of nausia. And so did I.

 

My question is what causes this sensation?

Also I have seen footage like this but have never got this sensation till I read this book. Weird huh?

[Edisonian]

I think our brains are wired that way to protect us from certain situations.