This will happen now and then, no matter how good your defenses are. Harpies can jump over walls and are very quick to run and infect buildings. Even with top notch defenses a harpy can break through and cause some damage. At the start a stroller might just slip past and disrupt things too. Here is what you can do to control an outbreak within your colony.
You can reclaim the area later
Before you panic, the game isn’t over. All of the buildings that get infected can be reclaimed and repaired once you have cleared all of the zombies in the area. The buildings are not lost. Do not worry about what gets lost right now, the focus should be on containing the outbreak.
Build some walls
You should already have a decent defensive structure in place to help contain this outbreak, but you should still build some new walls. These can be scattered around and do not need to look great. You will be deleting them afterwards anyway. They only exist for now as a way to slow the horde down, allowing snipers and other longer range attacks to pick off some zombies before they hit your next wall.
You will have a bit of time as the horde rips through any of the structures between them and your next defensive area. Withdraw all of your troops to the next line of defense and this should be your next place to mount a defense. Build any new walls you need as a temporary measure to buy you some time while you get your troops prepared.
Attack from behind with any remaining troops
If you have 3 or 4 defensive walls defending an area of the map and one of them falls, the troops in the other walls will not be attacking anything. Make sure you withdraw what you can and if they cant be withdrawn, group them together and attack the horde in whatever way you can. Yes, they may all die, but they are the price you have to pay to prevent the zombies from making it to your command center.
Sell Resources to buy more troops
This might seem crazy but this is a make or break moment. All the oil and iron you own isn’t going to be any use to you when the horde break through your final wall and are taking over your command center. Sell the oil and iron at the market to buy more stone and gold. Use this gold to build as many troops as possible. Replenish any casualties you can. You may be down on the number of workers that can support the building of new workers, but do everything you can. As I said, the resources are no good to you when the game is over. Blow through everything you can to survive and contains this breach.