I'm reading at the moment a very interesting book called 'Epidemic' by Dr Robert Baker, a consultant in microbiology at King's College Hospital in London. As I understand it (it's a very 'scientific' book, and I'm not that brainy!)) IF a virus jumped from animal-to-human, as with prions (BSE) and avian flu, it would be an entirely new threat which our immune systems had never encountered before. A second threat is the development of resistant bacteria which no known antibiotic can defeat, eg MRSA. Thirdly, a hostile country could artificially develop a virus or bacterium which had no remedy and caused invariably fatal results. A tiny quantity of Botulinum toxin (from a common bacterium) could wipe out the entire population of the Earth. I realise humans would be the active vectors, but these microscopic beasties really do have the potential to do away with us. Whether that makes them the 'dominant species' is a matter of opinion. It's all very fascinating though.