At about 20 minutes into the show, Chloe Sullivan displays an aerial photo of Metropolis on a computer monitor. The aerial photo is actually of the south side of Chicago Illinois.
Lois's outfit is similar to Jill Valentine's outfit in Resident Evil 3. Mainly the blue top and the black knee high boots.
At the beginning of the episode, Clark wakes up and finds Metropolis deserted. This bears a strong resemblance to the first scene of the film 28 Days Later. The appearance and behavior of the virus-infected zombies is also very similar to the "Rage"-infected zombies from the same film.
This is the third episode to feature a virus. The first was "Scare" where LuthorCorp's serum caused several townsfolk to be overcome by their inner fears. The second was "Thirst" where Buffy Sanders was infected by a kryptonite-based vampire bat rabies.
The term 'zombie' is only mentioned once when Clark & Lois are in the hospital visiting Tess. Lois: "Seriously? Alien orbs, now she's rambling about zombies?"