It almost seems as though the shrew is actually an unbelievably vile and abominable being created by The Ultimate Evil Itself (®, most likely) in order to eradicate all mankind. It probably gorges itself with all sorts of putrid waste until state of the perfect internal decay is achieved. The loathsome shrew then enters the last stage of its life cycle - masterfully placing itself in the way of some unwitting oaf so that it can be stepped upon and thus release the foulness it has accumulated within itself.
Yes, I can totally see it now.
Quite the ingenious plan, I must admit.