Gas goes around in different areas of the city, as the city is, the only way to survive the gas is to always keep your mask on until you get inside somewhere, never take it off outside. Different places with different people and survivors that did exist in parts of the city, but infected people did too, where they'll make you breathe in the gas too. Different gasses have different effects on a person's mind and body. There were cures spread around the city, where you can save infected people.