This is the first time I've been asked about the lore so I'm happy to give some details, however most of the story will be told though cutscenes later and may change slightly.
The virus is created by a rogue pharmacist who was trying to cure her son from a degenerative disease. Due to a series of events involving the police, an informant, an anti bioterrorism group and rats, the virus is spread throughout the city over a 24 hour period. The game takes place on the 24th hour of the outbreak, where 2 agents (the players) have been sent to track down an informant who has information on the virus.
At this point no one has really had time to react as the outbreak is fresh but throughout the levels you can find clues of what has happened and how certain groups of people responded.