Of the computer variety, of course!
8 Computer Viruses That Brought the Internet to Its Knees on WhoIsHostingThis? starts out:
…The first computer virus, called the “Elk Cloner”, was created as a teenager’s harmless prank, displaying random messages to users and played subtle tricks when the infected floppy disk was booted.…
It gets worse from there. To read the whole thing, click the above link; to preview the graphic,