They Came From Within (aka Shivers) (1975)
Director: David Cronenberg
Writer(s): David Cronenberg
Cast: Paul Hampton, Barbara Steele, Lynn Lowry, Joe Silver
Shivers (filmed as Orgy of the Blood Parasites; alternate titles: The Parasite Murders, They Came from Within, and Frissons for the French Canadian distribution) is a 1975 Canadian horror film written and directed by David Cronenberg. Cronenberg won "Best Director" at the 1975 Sitges Film Festival.
A scientist living in an apartment complex kills a girl and uses acid to destroy her internal organs, and then kills himself. While investigating, a doctor discovers that the scientist was doing experiments on the use of genetically engineered parasites as organ transplants. Soon, other people in the complex begin showing signs of carrying the parasites, spreading the things through wanton orgiastic abandon, and the complex begins suffering an attrition problem.