At one point Earth's immense population was sustained only through aggressive exploitation of the latest advances in nano-technology. While undeveloped land existed only in nature preserves, and even Antarctica and the continental shelves had been settled by expansionist groups, pollution had been minimized, and what little remained of the natural environment was in superb condition.
At the beginnings of the plague, as nanite reliant technologies crumbled, billions died on Earth alone. Chaos and disorder gripped the planet for years. It was at this time that the Purist faction gained dominance in the Sol-System.
Earth is now generally considered a rather nicer place for the change with a mere eight-billion inhabitants.