viral describes something caused by or of the nature of a virus and, in its now-dominant figurative sense, content that spreads quickly and widely from person to person, especially through 📝social media.
The adjective entered English in 1944, formed from virus and the suffix -al, and for half a century it belonged to medicine: a viral infection, a viral load. Virus itself reaches back to the Latin for "poison" or "slimy liquid," and only narrowed to the modern infectious agent in the late nineteenth century. The figurative sense — having suddenly become widely popular through internet sharing — is attested by 1999, born in marketing and borrowing its image from the way a computer virus copies itself from host to host. The idiom "to go viral" names the moment a piece of content crosses from its origin into that self-propagating spread.
What the word describes is a shape of distribution rather than a size. Something bought into wide visibility through paid placement is popular but not viral; viral implies that the audience itself carried it, each viewer passing it to the next. That makes it an outcome, distinct from 📝Viral Marketing, the strategy aimed at producing it, and from a 📝Viral Loop, the product mechanic in which users recruit users. No view count marks the threshold, which is why the word is applied as readily to a post seen by a hundred thousand people as to one seen by a hundred million.
"You want word-of-mouth? Do things worth talking about. That's it." [...] "viral isn't a number, it's simply farther than you can reach yourself." ~ 📝Scott Stratten, 📝'Alienate Customers' Keynote
My 📝Facebook Ads Prank went viral on 📝Reddit, and 📝Ghost Influence was founded on its heels. The lesson matched Stratten's: viral is just a reach greater than your own, carried by people who found the thing worth passing on.