Hans Wagner was the patient zero Alpha zombie of the plague
That's why he sealed himself in one of his safes, and why the safe was thrown at sea, Sebastian might have heard embellished tales about the reasons he entombed himself. Then the safe was found, put in Area 51 and opened, zombie Wagner went out of hibernation and bit the soldier who would become Zeus, and got shot/destroyed. Then it was decided to keep Zeus alive, and then Army of the Dead happened.
