Black Plague

In GE2, the Red Rain is a pandemic that spreads an incurable disease called the Black Plague. When one makes contact with the red rain or afflicted patients, he/she will be infected as well. The Black Plague spreads throughout an infected body by indication of black-reddish marks being left visible on skin. Several urban inhabitants of Anagura are infected wtih the Black Plague. There has been no cure for this plague and death is 100% guranteed for infected patients. Aragami do not seem to be affected by the Red Rain.

History
The Red Rain and Black Plague are actually part of a system to find a new Singularity after the prievious one (Shio) had left to the moon. Infected patients are "candidates" for this new singularity.

Additional Information
The Red Rain and Black Plague first make their appearance in the prologue manga, GE: the 2nd break which crosses into the GE2 plot and also depicts events that occured before the game.