Shivers (aka They Came From Within aka The Parasite Murders) 1975 Dir David Cronenberg.
The inventor of "body horror" first "proper" horror feature film (Crimes of the Future is more a experimental work) is the template for the first decade of his career. A isolated expensive apartment block in Montreal is the environment invaded by a man made parasite that is passed between humans with results best described as "ikky". Its a cheap film and one that suffers from some rudimentary standards in acting, sound, editing etc but also one that builds, as Cronenberg gains confidence, to a well judged climax as the inhabitants of the complex take on the aura of a sexualized zombie horde.
Shivers is one of several films made in the 70s I've seen recently which would almost certainly fall foul of censorship (or more likely a demanded rewrite) if made in exactly the same way now with teenage children at the core of objections. I won't give anything away but there's a couple of moments that would not be filmed as was. That said it was condemned at the time described by one critic thus - "crammed with blood, violence and depraved sex" and "the most repulsive movie I've ever seen." (some would say that's a rave review! )
Not one for the sensitive then.