Saw "Frissons" (1975)
aka "Orgy of the Blood Parasites"
aka "The Parasite Murders"
aka "They Came from Within"
Well, David Cronenberg´s movies are all a little bit strange - not a new insight.
But this one is relatively "normal":
parasite-worms crawl from human to human (by kissing/by sex) thereby increasing their host´s libido.
A whole apartment-block turns into sort of a "love community" - man and woman, woman and woman, man and man, father and daughter - in every possible constellation (often in mass orgies) the infected victims are having it off together.
The end is: everyone gets infected, even the good protagonists.
Hm... german TV calls this movie a horror...
Ok, some people are shot down or beaten to death in this movie, but these slugs/parasites do not kill anyone they just make you keen on other humans.
I would say this is not a horror, this is a manifest for the future: just do it!
Long live the slugs!!!
I have this on DVD, under yet ANOTHER title. The DVD is called "Shivers." It is one of my favorite Cronenberg movies. Interesting that you mention it, because I just watched the DVD again about a week ago, back-to-back with Cronenberg's "Rabid."
This is a very good film in my opinion. When I saw it a few years ago I found the film under the title SHIVERS. Worth seeking out. Also you have an older Barbara Steele in this one. Don't miss it !