Shivers (1975)

The feature-length film debut of legendary horror director David Cronenberg!

After a scientist living in a posh apartment complex slaughters a teen girl and kills himself, investigators discover that the murderer had been carrying on experiments involving deadly parasites. Roger St. Luc (Paul Hampton), a doctor living in the building, and his aide, Nurse Forsythe (Lynn Lowry), then realize that the parasites are on the loose, attacking fellow tenants. And those who become hosts turn into erotically obsessed maniacs who pass the bugs on through violent sex.