I’m looking for the name of a movie I saw a few years ago. I believe that it starts off with a high school girl (possibly called Katie?) breaking up with her boyfriend in a book or video game store. It is explained that at the start of high school they both didn’t have many friends and so they befriended each other but Katie(?) became more popular and so didn’t hang around with him as much.

After this the boy and possibly some of his friends spread an illness around the school (may have been to get back at Katies new friends) that I believe is contracted by transfer of saliva.

I remember that there were some scenes in a football locker room, some in the school cafeteria and also some at a house party.

Later on in the movie Katie’s ex-boyfriend kisses her to infect himself with the illness and then goes to some sort of party, possibly prom, at the school and eventually tampers with what I believe was either a chocolate or maybe just alcohol fountain and transfers the illness to it and infects everybody in the room.

I believe that the movie ends with Katies ex-boyfriend being arrested after the party at the school for spreading the illness.