The way the pilot unfolded, I thought the problem was that the drugs were tainted/bad, causing many of the junkies to die. The issue was that the girl came back first and fed, one stabbed her, but most of the others rose from the dead later, after the surviving junkie saw the girl and ran.
That explains why the building was pretty much empty but for another really scared guy when the stepfather came back.
An interesting twist could be that the zombie virus is spread through intravenous drug use like other things spread via bodily fluids. It spreads nationwide via the drug trade and drug smuggling, which is why it pops up all over the place AND goes uncontrolled for so long. The violence of people attacking and eating each other won’t stand in a suburb, but in a drug den or homeless shelter, people think it is “bath salts” or another bad high and let it go, ignoring people who rise from the dead until it is too late to stop another few getting infected. Until the strain goes airborne or somehow otherwise gets into the whole population.
Remember Jenner from TWD at the CDC? He told Rick that everyone is a carrier of the virus and that when everyone dies, they will too rise from the dead.