Warner Bros. animation was in serious financial trouble during the film's production, and in 2000 it came down to two options, green-light another film by Director Brad Bird who made the critically acclaimed film The Iron Giant, or this film, Warner Execs choose this film over Bird's and upon learning this Bird got so mad at Warner Bros. that he took his film with him to Disney and Pixar animation to be made, that film would be the highly successful The Incredibles, whereas this film bombed at the box office, was critically panned, and ultimately killed Warner Bros. Feature Animation.
When Frank's daughter decides not to go to Buffalo with him, we see the ticker above his door showing the scores of various sports games. The item "Seattle: 5 Oakland: 4" scrolls onto the screen in two consecutive shots.
Chill was sent into Frank as a flu vaccine. While he could have been a live, weakened virus had he been administered via LAIV (FluMist), this variety of the vaccine was not introduced until 2 years after the film was released. So Chill would have had to come from an injection, meaning he would be a dead virus.
During the beginning when Ozzy is patrolling the mouth in a helicopter, there is a shot where some of the bubbles of egg and saliva very quickly shift position. This is likely an animation error.