U3 is a USB flash storage app that lets you carry and run Microsoft Windows applications directly from the flash drive. For some people, this is a useful tool. However, it is also an extremely invasive piece of software that automatically installs itself on a Windows PC without asking permission or providing the option not to install as soon as you plug in the USB flash drive.
_is_ a virus because ScanDisk allows it to INSTALL without first ASKING the user if they want it. Software that modifies your computer system without your permission is a virus.