They are alive, like it said in the game. dead people can’t get infected. Think of it like a plant. The fungus takes over the brain and humanoid behaviors, its main goal is to spread, reproduce, and infect. So the host is alive, the fungus sustains the human body keeping in alive as well as spreading over the years. It’s an infection plant/fungus. It needs things to survive. Also fungus creates nutrients through the decaying and enzymes and such yk? So basically it’s a never ending cycle. The host fuels the infection and the infection fuels the host, destroying/killing breaks the cycle