The Defilement was a period of horrific loss as Cogitatio vermis ripped through the population of Earth. It was initially thought by the Martian government to be a new disease of potentially alien origin however intelligence quickly suggested that it was actually an alien invasion.
Timeframe
The Defilement started in the year 2563 and continued for years.
Background
On the anniversary of the end of the Venusian/Alliance war the Bah-Jurang Listening Post went dark. Six week later, an infected E.P.U Fast Scout traveled from the listening post to Earth kicking off the final days of life on Earth.
Aftermath
Planets became extremely insular, quarantining their populations in order to protect them from both death and the truth. Mars began executing plans of mass migrations out of Sol by creating three Seedships: Sparrow, Dvinalya, and Petierre.
Losses
While it is impossible to get an exact number, the entire population that remained on Earth are thought to be dead. Approximations put total loss of human life in the trillions.