It happened on December 25, 2001, right after the new century began.
It was the biggest and worst disaster in human history, "Apoptosis Crisis."

A virus called "apoptosis" begins to destroy human cells. 80% of the Earth's population is killed and the people who survive try their best to revive civilization.
A century has passed, and it is now the year 2099 in Tokyo. Something is happening in this big city, where many kinds of ethnic groups reside. The network junkies, called "jugglers," suddenly die an unnatural death. The cause of death is still unclear. However, it is apparent they were all on computer networks when they died.
Takuya Takahashi, who is a professional investigator, a "Runner," is asked to investigate the case, and he starts investigating this dangerous disease, without even knowing much about it?

[From Gamelex]