The city is invaded by a weird fungus like plant that has everyone sick, some of them are changed, they seem to be drawn to you, attacking you with whatever is at hand. All you know for sure is that you don't appear to be affected. As things progress you realize that everything you use is less available to you. In the game guns don't re-spawn. Health packs aren't available every three steps. Food offers different health levels per type. If you acquire a automobile in the game you have to keep it filled with gas or it stops working. Everything in the game would be real life cause and effects. As the game progresses you would find that if you kill the plants that are around the city, slowly the people start to recover and help you. Eventually recovering enough to discover that there are other plants around the globe. The effect of your actions before becomes apparent, anyone that you killed earlier in the game is now someone that you don't have to help you against the attacks of the plants now that they would be health. If you went on a killing spree earlier, you now have to fight a army of plants alone. A few people like you seem to be immune to the toxic spores, but it seems that the pods that are on the surface are just the start. There is some growing inside the plants much worse.