Reanimator can be obtained by performing special season deliveries in the port, as a reward for progress in creating an antidote, and also purchased in the in-game store.

How does it work? You use the reanimator and get a temporary buff, it can be seen both in the corresponding section with character's skills and next to the HP scale. If your survivor takes fatal damage with this buff, his health is restored to the maximum, but the buff disappears. The buff itself lasts only 4 hours or until the death of the character.