The Cat Virus was a virus that had an appearance of a green cat that was created by Megabyte. It was able to infect devices and spam cat videos. Some of the names Megabyte was thinking of for it included Trouble or Nuisance.
When the Sourcerer wanted Megabyte to provide a distraction in cyberspace, Megabyte suggested the use of the cat virus. After it was captured and contained in a cage, it barfed up a silver bauble which was actually an encrypted file belonging to Austin's father.
The cat virus was then handed over to Virusylum.
When Hexadecimal was captured by the guardians, she was locked in a cell with the cat virus, whom Hex took ownership of.