In the finished build, the player will be able to win the game by outliving the zombies
It's canon in the lore that zombies decompose with time. A mere 6-month period is enough for them to lose much of their strength and speed. It's possible that the final release will take this further and let the zombie population decline over time as they decompose to death. It may take decades of in-game time, but the player will eventually see a zombie-free world again, at which point the game will somberly note that civilization is still gone forever.The Knox Infection was originally two different illnesses.
Both the official explanation and the prion theory are correct. The element responsible for the zombification was a prion hiding in a meat shipment delivered to Spiffo's, a popular regional chain in the area, and was responsible for the first outbreak on its own. The second component was an unrelated but hypertransmissible flu-like illness that began spreading around the same time and caused flu-like symptoms. Prions are bad at spreading, however, so the outbreak was easily-contained. However, viruses mutate. One such strain of the virus infected people who had also been infected with the prion and the new, mutated strain of the virus became capable of assembling the prion, which is how the infection suddenly became airborne for the second wave. This also explains the transmission oddities - While it's possible to be immune to the virus element, which means you can't get sick from the airborne version and it can't assemble the prion, the prion is still just as dangerous and is transmitted via direct fluid contact.