Evilution (2008)

Something goes horribly wrong when the military uses an alien virus to reanimate dead soldiers. Right before the zombies attack and everything goes kaboom, a soldier named Darren flees with a vial of the virus.

With the military searching for him so they can retrieve the virus, Darren moves into a basement apartment in a crappy building in a gang filled neighborhood. He is repeatedly hassled by the gang because they used to hang out in the basement before he moved in.

The basement looks more like what you'd find in an industrial building rather than an apartment building, and the apartment is creepy and isolated. The landlord tells Darren to keep his door locked, but he keeps leaving it open. This is horrendously stupid, especially since the gang keeps barging in, and he has a vial of the zombie virus sitting out in the open on his make shift laboratory. Also at one point a junkie wanders in, thinks he's found a drug lab, and absconds with a needle full of virus.

When one of the gang is shot, Darren brings him inside and tries to save him by injecting him with the virus. Darren goes upstairs to a neighbors and when he comes back down the gang member is gone. Yup, the zombie virus has been unleashed in the apartment building and havoc ensues.