After the first mission you will learn to "cleanse" terrorist safehouse, the same safehouses Nick uses in the story. There are 8 total, in the same spots as the main campaign, and you need to walk up to an exterior vent and tap to use the larvae bioweapon you picked up from the helicopter crash site, turning the safe house icon from green to red. Enjoy the screams of pain and terror from within the safehouses once you use the bioweapon!
Make sure you finish this before going to the Mayor's mansion to avoid another playthrough.
This video guide shows all Safe Zones that require infecting to unlock this achievement. The ability to infect Safe Zones is not available from the start, but acquired later in the story, as seen in the video!