I am confused about intervening cause. I know that it alters the natural and continuous series of events that follows.

But, I can't decide what could be considered it... Like, is nature considered an intervening cause? How about Instinct? Sickness? Animals? I have tried and tried to figure this one out...

nature could be...

A woman stores her gasoline next to the house of the yard while mowing her sisters lawn. Lighting strikes, igniting the gasoline, the house burns. Her storing the gasoline is immaterial, the intervening cause is lightening.

Animals could be....
A person puts rat poision in the garage to kill mice. A neighbors cat picks up a dead mouse and takes it home to her food provider. The neigbor screams, falls, and breaks her back. The cat was the intervening cause, not the killing of the mice.

An intervening cause is an unforeseen event that causes damage to someone based on the prior actions of someone else.
http://law.freeadvice.com/general_practice/legal_remedies/intervening_cause.htm