Why does Frank west get infected?

Frank West gets infected in "Dead Rising" due to exposure to the zombie virus while investigating the outbreak in Willamette Parkview Mall. During his time there, he is bitten by a zombie, which transmits the virus into his system. However, he later discovers that he possesses a unique immunity to the virus, allowing him to survive while still facing the challenges posed by the undead. This immunity plays a crucial role in his ability to navigate the chaos of the zombie apocalypse.