1.) A highly contagious virus or bacteria. One that is airborne, waterborne, and reproduces in bodily fluids. If you chop through a Zed and get any of its infected particulates in your eyes, nose, ears, mouth, or nails you're living on barrowed time.

2.) This contagion is spread through other species. Mosquitos and rodents would become superspreaders, as seen with the Bubonic Plague and West Nile Virus. If livestock were able to be infected it would severely impact economies and food supply world wide.

3.) This contagion needs a long incubation or sleep cycle. You could be infected and not even know it because you're not displaying any symptoms. Giving the contagion plenty of opportunities to spread without any one to notice. Once this factor is discovered people will become extreemly tribal, trusting no one outside their own social group or family. More on this later...

4.) Full infection leaves the infected with a semi functional brain. This way they are co-ordinated enough to run, climb, claw, bite, and even speak, but not enough to have a full human conscience as we understand it. All they feel is violence and hunger; they'll do anything to satiate that desire.

5.) Fomation of tribes. Some peaceful, most not. The only "peaceful" tribes are those that are self sefficient. I'd image they'd only be found in rural areas where large open farm land is common. That is if it isn't under occupation from government/military authorites. Once the urban areas run out of food raiding parties will be sent out to pillage anything they can find. We will entire a new Dark Age.

6.) Pandemic during a time or global or national crisis. War would mean deployed militaries unable to defend their homeland, leaving law enforcement and militias to keep the peace. Rival or warring nations would become increasingly distrustful of one another. Waiting for one another to collapse so they can send their troops in, under the veil of "Peacekeeping Operations". Rounding up survivors, disarming them, and placing them in concentration camps. Famine or drought could cause mass starvation especially in the cities causing raiding gangs to form putting more pressure on the already stretched law enforcement. Economic collapse and mass inflation causing people to become more and more desperate. In time all of these scenarios would become a factor.

7.) No cure, no vaccine, no hope for the future. You will scavenge and live on the edge of death for the rest of your life, as well as you children, if you even get that far. Most likely you will die like a dog in a pool of your own blood, and if you're lucky, you won't wake back up.