From Yen On:
The World Has Already Ended. The future--where a terrible battle against a parasitic virus called "Gastrea" has been fought...and lost. Humanity is cornered and lives in despair. Rentaro and Enju face constant danger in their work as a team of anti-Gastrea specialists known as "civil security officers." As if the daily fight against oblivion weren't enough, they'll soon face a threat that could destroy all of Tokyo.