The Infected Citizens are Robot Citizens that have been corrupted by the Maelstrom.
After the Master NXT became corrupted by the Maelstrom, he spread his corruption to Robot Citizens, turning them into Infected Citizens. The Infected Citizens now run around the streets of Robot City, attacking players on sight. Until the rest of the Robot Citizens can set up a proper firewall, they are all susceptible to becoming an Infected Citizen. However, to contain the spread of corruption, Unit 16 sends players to smash three of the Infected Citizens, which each have 5 Life.