Shivers (1975)

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.—Ed Sutton