Contrary to what you might think, the term "viral" has nothing to do with computer viruses. Instead it refers to a digital video, image, or article that has spiked in popularity and has reached a large number of users in a short period of time. While there is no exact number of views that makes something "go viral," most viral media is viewed by more than a million people in less than a week.
Viral videos are the most common type of viral media. Most viral videos are posted on YouTube, which provides free video hosting. Since YouTube has become the central location to view videos on the web, homemade videos have the potential to be viewed by millions of people around the world. While some users have success promoting their YouTube videos from other websites, most viral videos gain popularity by word of mouth. For example, if someone comes across a video that he thinks is especially amusing or shocking, he might forward the link to his friends. If his friends also like the video, they might tell their friends, who may tell other friends, etc.
For a video to become viral, it needs to reach a certain threshold that might be considered a "tipping point." Once it reaches this level of popularity, the number of views or "hits" spikes upwards and the video becomes viral. This may happen when the video reaches YouTube's "Most Popular" list or is picked up and shown by a national news network. Once this happens, the already popular video gets even more publicity and goes viral.