- Paranoia Fuel: 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).
- Seasonal Rot: 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 psychic empathy powers!), the bland and irritating character of Nina, the relative change in character of Bob, and the lack of a proper resolution.
- Ship Tease: 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 suddenly sensitive and nice and then kisses her - except that's a Daydream Surprise and when we return to reality David is actually being even more of a dick to her than usual. Sorry, Caroline.
- Values Dissonance: 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.
- The Woobie: 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.