From Amazon:

In The Year 2889 is pretty good. After a nuclear war 8 people end up in an isolated valley saved by vertical convection from a warm lake that carries radiation away. Grandpa is in charge and carries a side-arm. Donna Reed's son and his brother arrive early and he falls in love with Grandpa's daughter. The brother gets infected and loses his appetite - never eats but has a craving for meat, spends all his time roaming the woods looking for rabbits. Alcoholic low-life and 60's psychedelic floosy dancer girlfriend arrive and add suspense to the movie. Pervasive is the lurking outside of Grandpa's son who is infected and looks like a monster and roams the woods spying on the girls as they bath in the pool. Movie is worth an hour and half every 15 years or so.

From IMDB:

AIP gave director Larry Buchanan the script for Day the World Ended (1956) to use for this film. The result is an almost line-for-line remake.