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.
WWII German Soldier in A Gas Mask. Hitler should be given credit for seeing that gas was not used during the European war due to his own disgust with chemical warfare. He had been a gas casualty during the First World War.