Germ Warfare is the third part of episode 10 of season 2. In this episode, Dexter's family comes down with the flu, and the boy genius tries his best to avoid becoming ill. However, this becomes a problem when Dee Dee enters his laboratory looking for a tissue.
Dexter has just put the finishing touches on his "greatest creation", which is a sphere that he claims will give him the power to rule the universe. Then, his Mom calls him down for breakfast. However, Mom is revealed to have the flu and sneezes all over his food. After Dexter runs out of the kichen in disgust, he runs into his Dad who is also sick with the flu. When Dexter has ran up the stairs to avoid getting sneezed on by Dad, he runs into Dee Dee, who has the worst case of the flu. Dexter rushes to his lab for refuge from his sick family. Suddenly, Dee Dee comes into his laboratory looking for her "hanky", which is actually a tissue that she accidentally left in the lab, which leads Dexter to put himself in a bubble to avoid catching the flu. However, his plan soon goes awry when Dee Dee sneezes onto Dexter's latest invention seen at the beginning of the episode, which causes Dexter's bubble to explode. The episode ends with Dee Dee finding her tissue, but Dexter, on the other hand, catches the flu that he was trying to avoid. A sick Dexter gives the viewer a sad look of defeat and the episode ends.
The three talking amoebas found under Dexter's microscope appear to reference Powerpuff Girl's Amoeba Boys (Craig McCracken was working on Whoopass Stew aka Powerpuff Girls at the time and the shorts were created and debuted the first appearance of the Amoeba Boys during the "What A Cartoon" run in 1992. However, their "official" appearance when PPG became an official series was in December 2, 1998 with the episode "Geshundfight".) During the timeline between the airing of this Dexter's Lab episode and the debut of The Amoeba Boys on PPG, both Craig McCracken and Genndy Tartakovsky were working on both series so the appearance is not coincidental.