Infection is the first chapter of the manga and the first part of Volume 1.
Taku Kamishiro, student in Class 2-3 of Kiyou High, spends his free time cleaning the school grounds. As the school day ends, students leaving the school stare at Kamishiro as he tries scraping off a piece of gum on the floor, to no avail. The school janitors finish up for the day and ask why he enjoys cleaning so much, to which Kamishiro replies by stating that he enjoys it for no particular reason. The school janitors bid him farewell as Kamishiro continues scraping off the piece of gum. Mika Hanazono, childhood friend of Kamishiro, suprises him by kicking him, causing him to knock over a flower pot.
The two chat while rearranging the flowers. Hanazono remarks that Kamishiro is always taking on cleaning duties even though he isn't supposed to and asks if he's a maid. Irritated, Kamishiro tells her that he has a lot of things to say to her. When asked what he wants to say, Kamishiro starts getting nervous and simply tells Hanazono that he'll meet her at the old school building during lunch break the next day. After punching him in the gut, Hanazono states that he'll make up some sort of excuse for not showing up tomorrow and leaves to meet up with someone. Kamishiro follows after Hanazono and sees her with Kouji Shinomiya, the good-looking captain of the football club with amazing grades who plans on confessing to Hanazono. He is rejected and left speechless. Kamishiro, relieved, decides to confess to Hanazono tomorrow as many other boys have their eyes on her.
Back home, Kamishiro talks with his sister on the phone and admires the couple of a romantic drama airing on television. He starts planning on how to confess to Hanazono while rubbing an urn clean at the same time. When he goes to clean the window, he sees a man scolding a woman out on the street. Kamishiro pretends he didn't see anything and walks away. On the TV, a celebrity states that her ideal man is someone who she can rely on even if he is a bit lacking. This motivates Kamishiro to confront the man out on the street. When he tries calming the man down, the man tells him to not come any closer or he might get infected as well. The man tackles Kamishiro with bleeding eyes, telling him to run away as he will be infected by the man's emotions.
Kamishiro wakes up on his bed the next morning. He gets ready for school and is informed by his neighbor Tanaka that a drunkard assaulted him and a woman. Not remembering the incident very well, Kamishiro wonders if the shock caused him to lose his memories. On the way to school, Hanazono greets Kamishiro by smacking him from behind. Kamishiro pays Hanazono no attention, preoccupied with thoughts on whether or not something has changed around him. He proceeds to plunge his umbrella through Hanazono's throat. Kamishiro comes to his senses and realizes that his murder of Hanazono was just a vision. With Kamishiro's arm raised towards her and his umbrella falling over, Hanazono is visibly puzzled. Kamishiro pretends he was just grabbing something from Hanazono's neck and is horrified over what he just saw.
In school, Shinomiya is upset over Hanazono's rejection of him the previous day. He vents his anger on Kamishiro and tells him not to get full of himself even though he and Hanazono are close. During class, the teacher sees Kamishiro making a scary face and asks if he is unwell. Kamishiro says that he was just thinking about cleaning and is laughed at by the whole class.
Kamishiro scrapes off the piece of gum from yesterday. He makes no mistake that he has an uncontrollable killing intent towards Hanazono, but decides to confess to her anyway. With a grim expression, Shinomiya sees Hanazono heading to the old school building. He confronts her on the rooftop, angrily asking who is planning on confessing to her. Kamishiro shows up and tells Shinomiya to move aside. After telling himself to express his true feelings for her, Kamishiro says to Hanazono, "I want to murder you."
Hanazono is visibly confused and Kamishiro tries to repeat himself. Irritated, Shinomiya confronts Kamishiro and is suddenly thrown against the rooftop railing by him. Kamishiro tackles Hanazono to the ground and strangles her, while being confused on why his body is acting on its own. Hanazono apologizes to Kamishiro for making fun of him and she only wanted to see how he would react to her. Just then, Kamishiro sees another vision of him killing Hanazono with a number of sharp and blunt tools. He loses control and dashes at Shinomiya, who is charges at him with a metal bar from the broken rooftop railing. Hanazono tries to stop Kamishiro and pulls his arm downwards. Kamishiro's fist crushes the rooftop and causes it to collapse.
Students and teachers hear the noise and rush to the scene. Among the debris, Kamishiro crawls to an unconscious Hanazono and tries to confess to her, but loses consciousness. Kamishiro narrates that he wouldn't have been able to confess to her if he died at that time, but the two could have avoided the tragedy that followed if they had both died that day. Hanazono opens her eyes.