means Describing content—a video, post, or meme—that spreads rapidly and widely across the internet through people sharing it, as if by contagion.

from From "virus," Latin for "poison" or "slimy liquid," once used by Romans for venoms and noxious fluids. Science later borrowed it for the tiny infectious agents that hijack cells to copy themselves. The adjective "viral" followed in the 20th century, and by the late 1990s marketers and the web had seized the metaphor: ideas, like infections, replicate by passing from host to host.