Leon is now best buds with the current President
Adam Benford is the man who first brought Leon in to help rescue Ashley, and now Benford is the President of the United States. Having been with Leon from the start of his time at the Department of Security Operations, President Benford is very close with the former Raccoon City police officer.
Thats why Leon is at the President's side in Tall Oaks when all hell breaks loose. After the C-virus outbreak infects a great deal of the population, including President Benford, Leon is put in a tough spot. With no other choice left, Leon kills the President before the mutation goes too far, kicking off Leons hunt for the people responsible in Resident Evil 6.
Sherry Birkin also survived the Raccoon City Incident
When the Raccoon City outbreak occurred, there were still some Umbrella scientists trying to preserve their research on biological weapons. Once such man was William Birkin, father to the very young Sherry Birkin. When things got their most dire, Dr. Birkin injected himself with the G-virus, turning him into a constantly mutating monster. Sherry was left to her own devices, hiding out in the city, until Claire Redfield came along, and helped rescue her from the madness. It's all very Newt/Ripley.
But she's infected with the G-virus
Before escaping with Leon though, Sherry was infected with the G-virus by her father. Once the U.S. government picked up Leon and Sherry, she was escorted to a secure facility where she could be safely monitored.
Thankfully, she didn't mutate into a giant one-eyed creature (puberty is tough enough for girls), but she carries the virus inside her to this day. Instead of leading a life of solitude under constant watch, Sherry chose to join the DSO so she could at least lead some semblance of a life.
Ada Wong is a spy who helps Leon...
Wherever Leon goes, so goes Ada Wong. First appearing during the outbreak in Raccoon City, Ada was a mysterious woman in red who helped Leon out of a tight jam or two. When Leon was in Spain, Ada showed up once more, this time to assist Leon and Ashley in escaping from Los Illuminados. Again, Ada reappeared in the East Slav Republic where Leon was investigating the black market sale of Bio Organic Weapons (Or BOWs, and you're going to want to remember that). She saved his hide from a few Tyrants, and disappeared back to wherever it is she goes during Leons downtime.
There's very little anyone knows about Ada Wong, except that she's awful friendly with Leon, and she always shows up at the most convenient times. That may have something to do with her ulterior motives.
...when she wants to
Remember how we said Ada is always helping Leon? Well that's only partially true. For every time Ada gives Leon a hand, she's got another holding the knife to stab him in the back. When they first met, it was under the auspices of Ada searching for her boyfriend. Turns out she was just looking to steal the G-virus. Leon even took a bullet for Ada before finding out she was a spy working for a mysterious organization.
In Spain, though Ada leaves Leon and Ashley a jet-ski to escape with (what a gal), that's only after she steals a Plaga sample from Leon at gunpoint. In the Eastern Slav Republic, Ada may have helped Leon escape (again), but thats only after she left him to fend for himself against a horde of Lickers and a few Tyrants. Even after all that though, Leon still has a soft spot for Ada. Which makes sense because shes super-hot. Whether or not she feels the same way on the other hand, remains to be seen.
Ada Wong is still working for a mysterious benefactor, and has another strain of the Plaga parasite
After the events of in the Eastern Slav Republic, Ada Wong escapes with a dominant strain of the Plaga. It's still unclear just who she's dealing with, but we can rule out Umbrella, Wesker, and TriCell, as they're either no longer living or defunct. Plus, Ada has double-crossed pretty much every single organization on the planet at this point, so even if we did find out whom she was working for, it wouldnt be uncharacteristic of her to suddenly switch sides.
Someone, somewhere is hoarding all these viruses, and with Umbrella out of the picture for the very first time (or are they?), it's still a mystery who is continuing to create all these horrible plagues. All we know is Ada appears to be their number one supplier, and it looks like she's up to her old tricks again in Resident Evil 6.
There are so many different kinds of viruses, you guys
Over the years, there have been a pretty insane amount of viruses created or modified by Umbrella. Starting with the simple T-virus, which led to the original zombie outbreak in Raccoon City, there have been more than a dozen different biological agents that have caused an outbreak.
Of course, as the viruses have gotten more potent, the creatures they've created have become more advanced and dangerous. Early on, there were just zombies. Then the viruses evolved, allowing the monsters to keep some of their intellect, or mutate wildlife into larger killing machines.
Even the mutated variants have gotten more impressive as the years have dragged on, and the viruses have been improved.
The viruses sometimes create unkillable monsters
While each virus creates a standard base zombie, certain genetic dispositions allow for the virus to mutate its host even further. Early in the series, infecteds like Tyrant and Lickers were common. The G-virus turned Dr. Birkin into like five different kinds of grossness, and the Uroboros virus turned people into massive, 100-feet tall monstrosities.
It seems the more advanced and unstable the virus, the bigger, badder, and tougher the mutations can become.
The black market for BOWs has exploded, starting in Eastern Europe
Even though Umbrella has finally been dealt with (or have they?), there's still a massive black market out there for bio organic weapons. Like most illegal trade operations, the BOW black market seems to have flourished in Eastern Europe. An entire civil war was fought in the Eastern Slav Republic with an army of Plaga, Tyrants, and Lickers. Though Leon is able to put a stop to the action there, he never does find out who was providing all the BOWs.
Not too long after the events in the ESR does another BOW situation arise in Edonia. Chris BSAA unit is dispatched to handle the incident, but things dont go so well. President Benford, seeing where the world is headed plans to organize the world in removing all BOWs from the picture, but his plans are cut short when the outbreak happens in Tall Oaks. Another incident happens in Lanshiang not long after that. Problems like this are the exact reason the BSAA was created in the first place.
Chris helped form the BSAA
Because of the huge increase in worldwide incidents of BOWs, Chris Redfield and Jill Valentine (among others) founded the BioTerrorism Security Assessment Alliance to help combat the BOW black market after the fall of Umbrella. Originally funded by the Global Pharmaceutical Consortium, the BSAA was their way of distancing the GPC from Umbrella, and it's nasty habits of repeatedly causing horrible zombie panics.
The BSAA has helped stop incidents aboard the Queen Zenobia and in Kijuju, both of which saw Chris on the front lines. The UN now controls the BSAA, which has stations around the globe. They are the first team to respond to the incident in Edonia, as well as Lanshiang.