In Black River Falls the non infected people could be around the infected people but if they were too close to them they could get it. So they used gas mask to be close to the infected people. It was an air born illness so that why they used gas mask.
One day, they hope to build a dome of sorts to give an entire city free pass to walk around with out a gas mask or any breathing mechanism on. Description from deviantart.com. I searched for this on bing.com/images