Usually, when playing a video game that has a clearly defined "hero" it can be assumed they are going to make it through most of the game's story in one piece. There are exceptions and ways that the developers can subvert expectations, but most of the time this is the case to some extent. A life-threatening situation can be a great tool for driving a story, and one very effective way to create one is to infect the hero with something early on. It might be a disease, a parasite, some sort of machine or poison, a curse, or some strange phenomenon, but the basic principle remains the same. The infection is used as a catalyst to set the hero's story in motion.