(Only based off the TV series, so pardon any missing info that debunks this) It would have to be some sort of airborne virus if it were to be worldwide within the time Rick was in a coma. It had to be a dormant or trigger-activated thing for it to cause so much chaos quickly because... [spoiler]Doctors would notice that people dying from natural causes/terminal illnesses/accidents were reanimating, and precautions such as quarantining near-death people and news alerts would be out before it became uncontrollable. The part of the virus which makes dead people just rise up would had to have activated all at the same time, or there would be more information available. Even if it was still uncontrollable, you'd think some survivors other than Dr. Jennings in the CDC lab would know earlier...[/spoiler] It also would have to have some lethal component in it, as a standard bite from a zombie would do nothing (other than hurt like hell, and maybe some sort of infection due to rotting. Noticeably the zombies aim for the neck on unaware victims--->quicker to kill (and stealthy, in some cases)--->faster to reanimate) As for how...scientists effed up a cure for something...or it's a conspiracy! It was planned all along, given the possible dormancy of the infection.