I think they know more about the ordinary flu and how & why & who it kills but the problem with this Swine flu is that it crossed the animal human threshold and could evolve in humans to be virulent and deadly.
The Spanish Flu pandemic of 1918 killed 20 to 100 million (estimated ) across the world young healthy people not just young children ,the elderly or sick and was devastating.I imagine that's the worry now and with air travel the virus can move all over the world in a very short time .