* CompleteMonster: Vascily Popov, once we find out that [[spoiler: not only did he synthesize the Miranda virus and dupe Daisy into spreading it as far and wide as possible, but he also dug up the body David got his Spanish Flu sample from and left it in the open so that birds could spread the virus and start an epidemic]].
* ParanoiaFuel: By the tankful; the series might as well be named ''101 Ways That Modern Biology Will Kill Us All (When It Is Not Too Busy Saving Our Collective Bacon)''.
* SeasonalRot: The fourth season is not as well-regarded as the first three, thanks to the ''far'' more out-there-speculative main plot (humans have a dormant pheromone-sensing Jacobson's organ, which if activated using a modified Herpes virus can give you what amounts to [[TheEmpath psychic empathy powers]]!), the [[TheScrappy bland and irritating]] character of Nina, the relative change in character of Bob, and the [[CutShort lack of a proper resolution]].
* ShipTease: Caroline is just about the only woman David has interacted with outside of a strict 'let's solve this epidemic now' context but has still never appeared to be attracted to. In a second-season episode, he has a moment of being [[OutOfCharacter suddenly sensitive and nice]] and then kisses her - except that's a DaydreamSurprise and when we return to reality David is actually being even more of a dick to her than usual. Sorry, Caroline.
* ValuesDissonance: For American viewers, the tendency for explicitly religious people on the show to be shown as dangerous radicals or otherwise a little nuts is jarring. That said, Carlos describes himself as a man of faith and he is probably the most normal, nice and well-adjusted member of the cast.
* TheWoobie: Amazingly, Bob Melnikov verges on this several times but never, ''ever'' crosses the line, partly due to good scripts and partly due to great acting.