How it all started (the zombie virus)
At the beginning of the movie, we can see an area being sanitised because there was a small leak at a nearby nuclear plant. After the scene in which a doe is accidentally run over, we'll see that it staggers to its feet and turns to reveal that it is infected.
So my theory about how it all started is that the doe is the first carrier of the virus. It got infected because it died in the leaked nuclear plant area. Just like in 'The Walking Dead' series, you'll get infected by the virus if you die, even if you are not bitten. Thus, if the truck guy didn't hit the doe, there wouldn't be any zombie situation (of course, if anyone else is not dying in the area).
This is just what i assume. I'm sure there are other theories as well :)
I love this movie!!!