Don't know why this never occurred to me before. There's an old horror movie by David Cronenberg that I think fits here.
They Came From Within, aka Shivers. The summary:
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In the fancy Starliner compound in an island near Montr?al, a mad scientist tests a parasite in the body of his mistress. He believes that human is an animal that thinks too much, and he develops a parasite to increase the violence and sexual desire of humankind. There is an outbreak in the condominium, with the dwellers becoming fierce zombies of sex.
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The best scene is when one woman infects another by kissing her, the parasite moving up one throat and down the other. This follows the first woman being infected alone, in her bathtub. No physical process, but tons of mental changes.