This article uses an in-universe perspective.
The Great Panic is the name given by zombie researcher and historian Max Brooks in his book "World War Z" to the time of mass hysteria, looting, random crime and chaotic evacuation surrounding humanity's realization of the Zombie menace. The Panic largely began when the infection started to overrun the population in mainland China. Refugees escaping from the blight in China helped to speed up the rate of infection in other nations. At the same time, the undead were beginning to outnumber the living in Africa and India.
Because of the level of denial that the USA was in over the nature of the zombie threat through the first winter, the public was unprepared. Zombies move more slowly or freeze altogether in winter, so their spread slowed down. The "Alpha Teams" of special forces commandos were able to combat the initial outbreaks. Most of all, the public was told that the placebo "Phalanx" was actually a vaccine against the zombie plague, which it was not. The combined effect resulted in a false "return to normalcy" in which the public refused to believe that zombies really existed, or that if some of it was true, it was under control. No large-scale warning was made, and due to the media-as-big-business culture in the United States at the time, many news outlets had treated warnings of the zombie plague as simply another disease outbreak like Ebola or SARS outbreaks of previous years. As a result, the first that many typical middle-class suburban Americans knew of the undead threat was when zombies came crashing through their living room windows. Soon, the number of zombies was increasing exponentially.
By May, a female reporter broke the news nationally that a new disease really was reanimating the dead and turning them into mindless cannibals, and that further, Phalanx was utterly useless. This, combined with Israel's "voluntary self quarantine" soon before, is when the American public (and much of the rest of the world) finally realized that they were truly facing a zombie plague. The result was mass hysteria, with many "wannabe Rambos" simply grabbing guns and shooting anything that moved: it was said that they actually arguably did even more damage than the actual zombies.
Many in North America attempted to migrate to northern Canada as North American and European cities began to be overrun. However, no planned exodus was organized, other than news channels urging viewers to "Go North!". Many people that managed to reach northern Canada were unprepared to camp there, and supplies ran low. Cannibalism occurred in several areas. Many had not expected the zombie crisis to last very long when they left in mid-summer, and when the next winter hit, (the worst on record, because of all of the particulate matter in the air from the fires of burning cities), large numbers were not prepared; an estimated 11 million people froze to death that first winter in North America alone.
The climax (and final curtain) of the Great Panic was the Battle of Yonkers, where the U.S. military suffered a tremendous defeat in a highly televised battle against thousands of zombies in Yonkers, New York, due to poor military planning. The disaster at Yonkers occurred in August, three months after the Great Panic began. The catastrophe had devastating consequences for the morale of the public, and effectively marked the end of a coherent American response for some time. Less than two weeks after Yonkers, the eastern United States were abandoned in a massive retreat by the military to establish a new defensive line at the Rocky Mountains.
The exact time that the "Great Panic" began varied slightly between different nations and locales. South Africa's Great Panic is stated to have occurred a few weeks before it began in the United States (which was directly caused by a single female reporter breaking the story that May). China's media blackout may have been able to contain news of the plague reaching its own population for some time, despite the fact that it originated there. Due to more isolated lines of communication, the Great Panic in Russia began about a month after it began in the United States.