Great Breakout

The Great Breakout was a period in history, around the year 5000, in which humanity spread out to the stars. This also meant the solar system became more busy, as large amounts of frontiersmen and pioneers got concentrated in the asteroid belt while waiting to spread out in the Galaxy.

The Doctor noted that the spread of humanity at this time was like a tidal wave...or a disease. He added that when humans get together in large numbers other lifeforms sometimes suffer. (DW: The Invisible Enemy)