The 1975 science fiction/horror film 'They Came From Within' (AKA 'Shivers,' 'The Parasite Murders' and 'Orgy of the Blood Parasites') is definitely not for everybody.
This early David Cronenberg film, which no doubt helped establish his reputation as THE director in the "body horror" genre, should appeal to fans of the zombie genre courtesy of a few of its tropes (a siege scenario, hordes of people driven by maniacal impulses). Game masters running zombie-based RPGs may garner a few ideas -- if they can stomach the hardcore violence and the subject matter.
The plot revolves around a high-tech (for the 70s') self-sustaining apartment complex that a smarmy marketing representative refers to as "a luxery cruise on land." A bizarre murder and suicide reveals a disturbing development - a parasite that stimulates the libido has infested the complex and is turning its human hosts into mindless rapists and killers.
The parasites infest people in clever ways, one of which involves horror film veteran Barbara Steele and a bathtub (I think you get the general idea).
The parasites were created to assimilate damaged organs, taking only a small amount of blood as they perform biological functions. The scientist that created them, however, strays from the mission statement of his research grant and instead tries to develop something that will put rational man more in tune with his natural instincts (one character describes the creatures as "a cross between an aphrodisiac and a venereal disease).
Cronenberg's intelligent script contains interesting ideas and black humor. But casual viewers be warned -- this movie earns its R rating honestly.
Cronenberg subsequently followed thus film up with the even-more offensive 'Rabid' -- a not dissimilar film that zombie fans may also enjoy.