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.
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.