"The Faculty" (1998)
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Concerned about Casey's suggestion that everyone at the school could potentially be infected, Zeke takes everyone he trusts to his house where he examines and experiments with Casey's specimen. He discovers that it needs water to survive and can be killed by his drugs because it is a diuretic. Zeke makes everyone take some of his drug to prove they are human and unaffected. ... Marybeth reveals herself to be the queen, having faked taking the test earlier.