The episode begins with a virus army on Gumball's fur. The leader of the army rallies the troops, giving an inspirational speech about how they will use Gumball's body as a stepping stone to their ultimate goal of infecting everyone in the world. As they charge towards Gumball's nostrils, the ground beneath them shakes. The cause of this sudden movement is Teri attempting to force Gumball to wash his hands in the water fountain. After learning why he refuses to wash his hands (as well as his disturbing method of showering without his hand getting wet), she is able to overpower him and finally clean his hands. The screams of the virus army as they are washed down the drain can be heard, but Gumball dismisses it as nothing. After the three of them leave, it is revealed that the leader of the virus army has survived, and he plots vengeance for his fallen comrades.
Later that day, the class does computer work at the library, though most of them use the time to play Space Invaders. Gumball and Darwin, unaware that Teri is right next to them, begin to mock her obsession with cleanliness. She announces her presence, angrily, causing the two of them to apologize. Then she uses Gumball's computer to show them a picture of a finger that hasn't been disinfected after a paper cut. Gumball and Darwin reel back in horror at the disgusting picture. Darwin, suddenly afraid of germs, grabs some of Teri's disinfectant substance and rubs it all over his body. Gumball, on the other hand, remains nonchalant about germs, mocking their fear of it and dismissing germs as a part of life. Teri counters with the fact that keyboards generally have more germs than a toilet seat. Gumball sighs and begins licking the keyboard in a show of defiance. Unfortunately, the vindictive virus was on the keyboard, waiting for an opportunity to infect Gumball, and his exposed tongue was just what he was looking for. He gets sidetracked by the various bacteria that were also on the keyboard. Unwilling to let them infect Gumball instead of him, he fights them all off. Just as he finishes the last one, Gumball finishes licking the keyboard, causing him to scream in exasperation.
The scene cuts to the science room, where Gumball is using a microscope to check Darwin's butt for hair. He doesn't find any, concluding that Darwin will never grow a beard, which saddens him. Teri again chimes in, telling them that an average butt has more germs than a toilet seat. Gumball explains that he isn't scared, because he never gets sick. He goes on to explain that he can make his body do whatever he tells it to. He demonstrates this, rather disturbingly, by performing several movements considered biologically impossible for a vertebrate. After convincing Teri of his fascinating ability, he collapses in pain and is brought to the nurse.
At the nurse's office, the school nurse explains that Gumball's organs are scattered throughout his body, but he is otherwise healthy. Teri is visibly and audibly flabbergasted at the nurse's outrageous diagnosis. The nurse believes she is talking about the treatment she gave Gumball, but Teri is actually talking about herself. The nurse, annoyed with Teri's frequent visits, recounts all the times she panicked when nothing was wrong with her. Teri, having no rebuttal, simply sighs in defeat.
Back at the science lab, Gumball and Darwin continue chiding Teri, much to her annoyance. She angrily rips a hair off of Gumball, innocently explaining that she just wanted a hair sample. Upon examining said hair sample through a microscope, she discovers a deadly, malevolent virus. Gumball guesses her thoughts, and he and Darwin give her sarcastic looks. Teri pleads for them to at least look at her discovery, and when they refuse, she pretends to cry. Gumball, falling for her sad eyes, obliges her. He looks through the microscope and, to his surprise, finds out she was telling the truth. Darwin looks at it as well, and witnesses it evolving to grow wings. He relays his sightings to the group, causing Teri to panic and put on a facemask. Teri urges them to move to a sterilized environment.
The "sterilized environment" ended up being the Watterson household. Gumball gets on his laptop and looks up a cure to the virus, but Teri thinks of giving up, explaining that viruses are able to evolve to adapt to any situation, thus rendering any attempt to stop it useless. Darwin, remembering the old phrase "fighting fire with fire," begins to attempt mutating himself, with disappointing results. Suddenly, Gumball gets an email from a website called "FreeOnlineMedicationWithoutPrescription." Thinking he's found the answer to this problem, he excitedly opens it. Unfortunately, it turns out to be the virus. It takes over the computer and starts taking over all of the house's electronics. He uses them in an attempt to kill the trio.
Gumball, Darwin and Teri escape into the backyard, and narrowly dodge a saw thrown at them. Chased to the streets by a lawnmower, the trio catch their breath and assume they are safe, being away from all the appliances. However, the lawnmower touches a car, and the Virus mutates again and takes control of multiple cars.
The trio is chased into an alley, and are backed up against a wall. Gumball challenges the Virus, calling him out on his cowardice for attacking three kids with five cars. The Virus is offended, and deactivates the cars. The Virus approaches the trio, giving a long speech about Gumball's demise. While the Virus taunts, Gumball merely steps on him, ending the episode.
In reality, bacteria are larger than viruses, but when Gumball licks the keyboard, they are smaller than the Virus.
This episode aired in Spain before an airdate was even announced in the US and UK.
This episode aired in the UK before the US.
It was revealed in this episode that Teri's mother is a doctor.
The Virus' threats to rid the world of all living beings is a reference to the fact that viruses are not living things.
This episode was the most viewed in Season 2 with 2.570 million viewers, surpassing even the season premiere.  It is also the most viewed episode of the show as a whole so far. 
Based on what the School Nurse said about Teri's false alarm sicknesses, this episode takes place on a Thursday.
The wrestling tape seen in "The Prank" and mentioned in "The Club" can be seen on top the TV.
This is the fourth time we see Darwin's butt. (The fifth time if you count the trailer.)
This episode is often confused with "The Joy," as both are about a virus.
This is the second episode in which a character dies on-screen. In this case, it is the Virus. The first time was in "The Picnic."
The game all the kids were playing on the library computers was Space Invaders.
"Mumu," the GPS seen in one of the cars, is a reference to TomTom, a Dutch GPS manufacturer.
At the beginning of the episode which shows the viruses on Gumball, Gumball's fur looks similar to Grover's fur from Sesame Street.
The Virus' statement "I hope your body is ready" is a reference to a meme, and a quote from Reggie Fils-Aime.
In this episode, it was shown that Gumball is weak enough that Teri is able to effectively grab, restrain and hit him in spite of being light-weighted paper; while in "The Party," Gumball was strong enough that a slap from Teri didn't faze him at all.
When the Virus transfers from the laptop into the electrical system, Teri is shown being as short as Darwin.