The exact origin of the virus is unknown; many characters offer different hypotheses to its exact origin. The first reported case the U.S.A. heard of was in South Korea from Camp Humphreys. Israel also intercepted reports from India and several other countries that went dark during the outbreak and could have been the source of the plague.
World War Z zombies are fast undead creatures with a pack mentality; groups will chase a single target on sight to infect them. Their behaviour is heavily based on animal behaviour (hence the opening credits of the film which shows various animal species and their habits) -- for example, they are able to climb atop of one another to form a "pillar" of zombies to allow them to cross a high wall, a characteristic taken from ants. The zombies also behave like viruses themselves, as their only interest is in spreading the infection -- they have no interest in consuming or even attacking their prey; once they manage to land a bite on a victim, they will lose interest in that victim and immediately chase after another person. While they will grab at a victim they are very dependent on their jaws; snapping their jaws at those they hunt. At first they look like recently deceased bodies with pale eyes and black veins on their bodies. After some time, they degrade and appear more like stereotypical zombies, though they are still fast.
The zombies are driven to infect any human they see on sight and are attracted to any loud or man made noises; such as a car engine or a person's voice. When not attacking they enter a dormant state; listlessly standing around awaiting stimulus. The zombies secrete a black tar-like substance from their mouths. The ooze is surmised to be loaded with the virus though it has an incredibly short life span and must be directly injected into the victim through a bite. Having the substance splashed into the mouth does not result in infection.
Curiously, the zombies only go after those who are healthy. Those who have been seriously injured or stricken with severe illness are ignored by the infected. For example, Ellis, a soldier at Camp Humphreys, was ignored in a room full of infected due to his lame leg and was able to escape the room and incinerate the infected. Vast herds will actually avoid a seriously ill individual when hunting targets; running around the individual in their path. As a result of this particular quirk, the WHO was able to concoct a vaccine that renders individuals effectively undetectable by the zombies, by tricking them into thinking the person has been infected by a serious disease.
The pathogen is one of the fastest recorded zombie infections with people transforming in as little as 12 seconds. Once bitten a victim appears to suffer a violent seizure; twisting and contorting violently before being completely transformed. Amputation will stop infection; rendering the host unsuitable to the pathogen as a result of the trauma of the amputation.
In the film the outbreak of the zombie plague takes the world by total surprise with mass outbreaks occurring across the globe. Though there had been reports of small outbreaks beforehand they were dismissed or mistaken for smaller threats.
Former UN investigator Gerry Lane and his family are caught up in the outbreak in Philadelphia and narrowly manage to escape offshore to a survivor naval fleet. Gerry is sent by the surviving government to investigate a possible outbreak; his family's safety being used as a bargaining to chip for his compliance.
During his investigation Gerry notices and meets several people the infected avoided. Ellis; a lame soldier, an old man in Israel and an emaciated boy who a pack of zombies actively avoid.