The virus appears in and is the main antagonist of the episode "Karen's Virus."
The Virus is a bright red spiked ball creature with a tail with a spiked ball at the end. He also has yellow eyes and sharp white teeth. He seems huge when inside Karen, but the end of the episode reveals he is actually a lot smaller than Plankton in real life.
Role in episode
Karen gets a virus from her friend Hanna after she sneezes in her face. The virus makes her delusional and hungry, and she runs around causing destruction in the city. Eventually, SpongeBob is able to confront the virus and learns that he's just hungry. Therefore he gives him a Krabby Patty.
He bears a passing resemblance to Swayzak from Toonami (both are computer viruses with deep voices and are red).
His voice sounds like Uka Uka from Crash Bandicoot, who was also voiced by Clancy Brown.
The virus isn't exactly evil; he's just hungry.
This is similar to the Courage the Cowardly Dog episode "Hard Drive Courage," in which the Hard Drive Virus infecting Courage's computer is only evil because he's sick. SpongeBob gives the virus a Krabby Patty, just like how Courage gives the Hard Drive Virus some of Muriel's gelatin with vinegar.
In addition to this, the virus here looks like the Hard Drive Virus.
He is one of the few characters in the series to be computer animated.