A Cloud of Madness — Bob Lock

A rogue comet traverses the orbit of Earth bringing with it a microscopic alien life-form that changes the processing power of the human brain and throws civilization back to the stone-age.
Human beings are no longer at the top of the food chain and therefore another entity takes its place. The future belongs to the robots but even they have been influenced by the strange life-form and their laws of 'do no harm to humans' has been corrupted.