I think its a "Contagion" Plot. In the 2011 film Contagion a virus spreads rampant killing 2.5 million in the U.S. and 26 million worldwide in 26 days. At the end you learn that a sick bat was eating a banana in a pig farm in a unnamed jungle. The bat flies over a pigpen, dropping a chunk of banana, which is then eaten by a piglet. The virus then mutates in the pig. A chef in Hong Kong buys the piglet. As he handles the carcass in the kitchen, he does not wash his hands when called to meet a customer. The chef shakes hands with the customer, giving her the mix of bat and pig viruses.
I think this is how it started.
The military was running test on the virus that hit Raleigh, North Carolina. When somehow the virus came into contact with one of the security dogs, mutating the virus (On the radio a janitor talks about how he works at a military base and one of the dogs started to get sick). The dog then transmitted the virus back to a human (Likely one of the security officers) where it mutates once again(In game if you get bit or scratched by a zombie it can take anywhere from 1 hour to 2 days for you to "start" to zombify, how long it takes for you to die can vary even more). When the military tried to stop the virus they thought it was the "Raleigh virus", but in reality it was a whole new virus. The "Knox virus". At one point they would have investigated how the virus got lose likely finding the (Now sick) security guard and his dog. And concluded that dogs were spreading the virus hence why they start taking dogs from people leaving the zone.
You learn that the owner of the Twiggy's bar in West Point left the night before the quarantine to visit a cousin and that he took his wife and dog with him. And that when he came back the next day the zone had gone up (Let's call the day the zone went up day 1). And that his dog had just started to get sick. You also learn that the World Health Organisation grounded all flights in and out of the U.S.A almost 8 days after the "exclusion zone" goes up. A study said that the average person will physically meet 10,000 people in one year, divide that by 365 you get about 27 people you will "physically " meet in one day. That means if Twiggy come into contact with 27 people every day for 8 days you get 216 carriers now lose in the U.S.A before planes are grounded. Now 27 sounds a bit high so lets cut it by half to about 13 rounded down. now it is about 104 in 8 days. Now what if those 104 people each came into contact with a human being over the course of 8 days. Let's do some math.
Day 1: 1x13=13
Day 2: 13x13=169
Day 3: 169x13=2,197
Day 4: 2,197x13=28,561
Day 5: 28,561x13=371,293
Day 6: 371,293x13=4,826,809
Day 7: 4,826,809x13=62,748,517
Day 8: 62,748,517x13=815,730,721
This means there is a potential 815,730,721 carriers lose BEFORE the planes are grounded (This is not even including the wife and cousin). Now, this is still a LARGE number but if you compare it to the population of Louisville 256,231 (according to a census taken in 2000). And the fact that Louisville is also home to the Louisville International Airport, that has 1400 domestic flights and 1605 international per day (2012). This could explain how the virus got around the globe even before it went "Airborn" (More on this later).
Now Outbreaks are confirmed around the world at around day 13. it takes about 2 days before you start showing symptoms, in the game if you take antibiotics and keep eating food so you stay at max healing you can stave off the virus. So if it takes another 2 days for the virus to kill someone with that would put us at about day 12. As for that last day the military kept the "Knox event" a secret for around 9 days. That is when the photo showing the first zombies got out. Other governments around the world could have done the same thing to try to stop the spread of panic. Thus easily putting us in the time-line of 13 days.
Now for the "Airborn" part. It took 11 days for the infected to escape the "exclusion zone". When the human body starts to decay it starts to release gases. it takes about 4-10 days for a body to start making this gas. When the undead escaped the zone and the military shot them it was like shooting hundreds of balloons filled will gases that could have contained the virus.
As for why the military set up the zone so fast... It was to save face. From the little we hear of the president he is someone that cares about his face. On the T.V and Radio. you learn that the budget for the CDC has been cut again and again after the "Raleigh outbreak" likely by the president. When the planes are grounded you hear the president say and I quote: " This move is as unwelcome as it is economically unsound.". In another statement he says " I assure the American people that every dollar I have at my disposal will go toward their healing and rehabilitation.". So we know he cares about money. If another a outbreak were to occur after he had cut the CDC budget again and again and the virus escaped a "GOVERNMENT LAB", all the blame would land on him. So as soon as he hear that there "Could" of been another outbreak, he told the military to block it off and cut all communications in and out of Muldreough and West Point. Also at around day 9 every radio and T.V station is down for the whole day with only static and garbled words. Hinting that the government tried to stop them from reporting on the infected escaping the zone.
One last thing is if we can say it takes around 4 days for someone to die to the virus then if we look at the chart above by day 4 the first zombies should appear at places not in kentucky. And by day 8 there should be between 28,561-371,293. So why do we not hear about zombies around the U.S until day 13-14? On the radio you hear about Riots across the U.S People raising guns to the police. At around day 5... Who wants to report about some old lady dying of the flu when you have the military BOXING in U.S citizens on U.S Soil. when you have RIOTS in the streets! Police SHOOTING UNARMED CITIZENS!!!!! Who wants to hear about a few sick dogs?
Same for around the world, think about 9/11. News stations all around the world were talking and covering that. And like a said around day 12 it is reported that zombies are now global on day 10 all stations start talking and showing the first video of the infected and covering the deaths at the "exclusion zone". And on day 11 the virus goes "airborn" so, makes sense why foreign nations would have all eyes on Knox and not outside there own doors. Also if we use something like the Walking Dead T.V show. According to the lore it takes almost 5 days after the first zombie before they become wide known.
But that is just a theory... A GAME THEORY! Tell me what you guys think.
One thing i don't have an answer for is some people in game talk about a "bad smell" that has been going on for "weeks" that would place it far before the outbreak. On the radio and T.V you hear about how there have are toxins in the fields and forests around the dixie area that have been there for almost a year so it could be that.
But ya tell me what you think and if I missed anything.