(I'm a writer at heart, hold on to your fedora.)
In the year 1941 on December 7th, the United States set up for the largest gamble in history, they would leave nearly half of their navy undefended at Pearl Harbor. They had complete faith in a system that was still in it's infant stages, RADAR, hoping that if an Axis alligned enemy decided to attack they would see them coming and react quickly. They were unable to account for the low skill of those in charge of the system. Despite detecting enemies numerous times, no one was warned of the impending danger and Pearl Harbor was more of a massacre than a sound strategy to find an entrance to the war. Luckily, this still opened the hearts of many Americans and soon they would be completely ready to wage war against the Axis. In 1944 the D-day operation was carried out and in combination with the Russian bear on Germany's other front, put a swift end to the Nazi regime. There was still one final enemy, Japan and Hideki Tojo. After their attack on Pearl harbor, the japanese military began to work on "weapons of mass destruction"; in the case of the US, we were nearing completion of the first atomic bomb. Luckily, the atomic bomb was finished before the Japanese were able to unleash their weapons and the war was ended in two devastating explosions. The Japanese weapons of mass destruction were left in tact, along with all of their horrid experiments. Instead of convicting the mass murderers, the US decided to recruit them. The United States was terrified of the communist threat. Communism was our next target and now that they had seen our most powerful weapon, we had to act quickly. They mass produced atomic weapons and had a research team dedicated to other weapons as well. This research team had researchers from the "Unit" tests in Japan among them.
(This should be a historically accurate review up to this point, I've mostly assumed the time period but this all works out. I've based most of this off of the radio types in game--although in terms vehicles, I'm really not if this will all add up.)
Long after the events of World War Two, in 1990, the cold war was nearing a close with several smaller wars in sattelite nations being fought and the USSR seeing the full extent of US nuclear power. However, in the case that nuclear war was inevitable, the US had a trump card. An experiment in secret, codenamed "Zomboid". Chemical warfare was the first option for the US, nuclear war could completely destroy the world after all. Only one problem, it couldn't be a physical weapon. No, this weapon must be kept under a guise of it's own. It would need to be a disease, something that was transmitted naturally. When the weapon was finally perfected, only one problem remained, the USSR would immediately suspect the US. So it would have to be an epidimic in the US as well. One of the leading scientists suggested a nearly full-proof plan. Quarintine one state ahead of time, military personal fully prepared. Transmit the virus through some means (likely vacination, as there are no infected items in the game that I know of.) and then wait for the entire state to become infected, it would be completely contained, but the media would blow it out of the water and the USSR would surely know about it. The US would then be free ro transmit the virus in the USSR as well, and with them being far less prepared, they would be severely weakened. The plan went through, and those involved in "Project: Zomboid" would never live to tell the tale.
Sidenote: I would think that the game must, by this theory (for the record this is a very subtle way of explaining a lot of epidimic games such as this, chemical warfare was incredibly common among our enemies in ww2.) take place around 1990-1991. Hopefully all the car models in game can attest to that. I hope most people think this is a relatively well thought out theory, I personally think it explains everything the best, especially the common use of radios. The only real hole in this is that the Soviet Union isn't in any of the radio broadcasts or TV programs that I know of. Perhaps this is an alternate reality where the "Red Scare" never happened so the US had to covertly destroy communism. You tell me. Also there might be a lot of errors, I didn't check over this at all, if it's possible to edit on this forum I might go back and do so for clarity.
