I think the first campaign would be Leon's, how the outbreak started and all. Then he and Helena get blamed for it so they had to fake their deaths and go to China like in the trailer. Then they meet Sherry and Jake and the game continues with you controlling Jake now. Then at some point they meet Chris' team and then you finish the game as Chris. After that, Ada's campaign is unlocked which lets you see how the background of the things happening in the game starting from how it all started and how it came to be at the end just like in Separate Ways and Ada's Assignment in RE 4. Honestly I just can't wait for this came because they're finally meeting together!
Well, the C-Virus is initially unveiled/spread in Chris and Jake's scenarios in Edonia, Serbia six months prior to the Tall Oaks outbreak.
There are three main campaigns, with a total of six main scenarios:
Chris Serbia Chris China Leon USA Leon China Jake Serbia Jake China
The best way to play the scenarios in the most chronological and story fitting order I've assembled from trailer analaysis and the developers words(they recommend starting with Leon and ending with Chris), is:
Chris Serbia Jake Serbia Leon USA Jake China Leon China Chris China
seems to be the best way to play the game (or possibly switch Chris Serbia and Jake Serbia since Jake is a survivor of Edonia's C-Virus test experiment, and later the BSAA would infiltrate). The Serbian scenario ends with Chris and the BSAA cleaning up the whole mess, and then admitting Chris into a hospital from which he breaks out of and starts drinking.