For several months before that everything was still fine, but because of an accident in a lab, they accidentally released someone who had strange symptoms and that person spread it through a bite. If a person is bitten then the person only has a little time before he dies but his body is controlled by something that makes him undead, the virus spreads quickly due to several other factors such as people who deliberately spread it or move people affected by the virus to areas where there is no such thing. The virus creates a lot of chaos plus they are difficult to kill and can only be killed if they behead or shoot right into the head so that it is completely dead and does not move anymore. People who are affected by the virus and become moving corpses become quite sensitive, especially when they hear the sound or smell of blood, they will look for the source, it makes it difficult for survivors to deal with such things and where they also have to look for supplies or may fight with others.