How does the character avoid getting infected in zombie games?

[strongdude]

Zombie games usually revolve around some kind of outbreak that may lead to people getting infected with some kind of virus that converts humans into zombies. The zombie virus usually gets transmitted through various ways. However, getting bitten by zombie is a common way of getting infected. However, in many cases, the character does get bitten by zombies. However, they never get infected by virus. So, what are some theories behind this?