THE DISEASE
Origins
Nobody knows the exact origins of the disease, but it had been kept dormant in the thick layers of ice on the North Pole. But whilst it had been trapped in ice for many years once the ice started melting it was only a matter of time before it was released into the atmosphere.
The disease hits
It didn't take long before the first human was infected, but it would take a long time before anybody knew there was a pandemic on the loose. Its incubation period was long, long enough that it would spread around the world before anybody noticed it. The weird thing was that the incubation period didn't seem to be time-specific, more like it was waiting for some sort of trigger, as it erupted all over the world at once. And those infected the day before got sick just as soon as those who had been infected early on.
At first nobody made the connection between the behavior of the infected and a disease. Since it attacked the mind first destroying the ability to reason properly bit by bit, making its victim prone to aggression, rage and lack of morality its behavior was very similar to criminal behavior. And since these were times of great unrest, due to political instability and the extreme weather that was haunting the planet, a wave of crime wasn't that unheard off. There were some that were affected in another way, appearing like on an acid trip or other drugs. They were just called a new wave of hippies, also not that unheard of in times of trouble.
Soon, however, it was crystal clear it was a disease. The disease started attacking the body of the infected, not unlike leprosy, and the mind deteriorated quickly, making some like crazed animals attacking anyone in sight, while others turned inwards themselves and was lost in a world of their own, completely unaware of the world around them. The infected were called zombies by those few not infected.
At this point there was widespread panic, and mobs quickly formed when there was suspicion of infections. Many escaped up into mountains, into forests, anywhere where they could isolate themselves. These were often dangerous as they would shoot anyone on sight in fear of the disease.
The second wave
While the first generation of the disease spread through the air the second generation had some new tricks up its sleeve. Spreading through the air required a certain amount of people in a certain amount of space to be effective, especially as the disease seemed to weaken once it started to spread from human to human instead of from the original source. So after a while the first generation of the disease went dormant, making short comeback where people gathered in large enough numbers.
The next generation was helped along the way by the nuclear fallout, both from ruined atomic power plants and from the nuclear bombs used on the big cities in an attempt to stop the spreading of the disease. This speeded up the mutations, and suddenly the disease came in many forms.
At this moment the biggest problem is the mutation that infect through the eating of meat. Even though the disease didn't affect animals, a lot of them were carriers of the disease. Eating some species was pretty much a guarantee for getting the disease, while other were completely safe. Then there were those that could be either. The problem was that nobody knew the disease traveled this way, and the lack of food made people desperate. They would eat any animal that crossed their way that they could hunt down and kill.
Immunity
Some people have a certain degree of immunity against the disease. This was due to everything from general health condition to what diseases they had been exposed to in the past, and what kind of disease they hadn't been exposed too. Some diseases seemed to offer up a bit of immunity, but most didn't. In fact, most diseases just lowered your chances of fighting off the disease.
Even though the military didn't seem to think a vaccine or cure was possible, there have been reports and rumor of groups or individual trying to find a cure or a vaccine.
The disease as the boogeyman
Everybody lives in constant fear for the disease, and when someone is suspected as being infected panic often follows. This would lead to mobs being created, and the suspected sick being killed on sight.
Although still a real threat, the disease is not as prominent at the moment, due to the low number of humans. There are bursts here and there which seems to be random, but at the moment the disease seems to be lingering about.
Although crime is the dish of the day, in a society where the strong will take what it wants and the weak will perish, criminal behavior is also an indication of being infected. This can lead to random killings in gangs that have formed when they think one of its members are acting a bit differently.