The Abomination (1986)
A middle-aged woman is diagnosed with a tumor by a charismatic preacher. She vomits up what she believes to be the tumor. The "tumor" turns out to be a strange spore of sorts that can grow into a carnivorous creature. This spore inside the boy's body now makes him brutally kill people to feed the other spores, which grow into strange creatures as described. - IMDB
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