Zombie Cliché Lookout:

Zombie bites generally come up during either the first or third act. Early on in the story, the victim is usually a minor character whose death/zombification is used to help lay the ground rules. They show us how serious bites/scratches/fluid transfers are, and help give zombie battles a bit more gravity. If it’s later in the story, expect someone close to the protagonist(s) to get infected. This is typically an emotionally charged moment that demonstrates how much a particular character has changed since the beginning.