We already know Helena Harper has something to do with it. Im guessing she is a spy for the US govt and manages to get her hands on a sample of the virus. She was probably blackmailed by an unknown antagonist to release the virus in order to save her family/someone important. Now the mission is to escape the city aka racoon city all over again.
On a side note, doenst it take a few hours for the infected to turn into a zombie, and i believe they already have a vaccine for the virus, so i dont get why they couldnt have prevented it.