Originally coined as part of the phrase viral marketing the term "viral" has expanded to mean any form of reoccurring practice that moves a product around from person to person. Because it is easy to copy and forward information on the net, anything that piques netizens' interests may get passed around very quickly. For example. if someone on YouTube posts a funny video, and the next thing you know you''re viewing it with a group of people at work, then it's "gone viral."
Historical perspective: On December 21, 2012 PSYs "Gangnam Style" became the first video in the history of the Internet to surpass one billion views.