What's the Name of That Book??? discussion

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Maniac
SOLVED: Children's/YA
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SOLVED. MG book about a supernatural force that 'infects' people and eventually drives them to suicide [s]
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It's about a girl whose high school teacher accidentally brings back some kind of supernatual force/entity/thing from an overseas trip. I don't remember how the students came into contact with the thing originally - the teacher might've brought whatever contains the supernatural thing to school to show them. From there the thing finds a student at night, and they usually have a dream so vivid they believe it's real, but is actually the supernatural thing "infecting" them. When it infected them it left some kind of mark or symbol on them, I think.
One of the victims was a "Nerdy" boy who hung around the female protagonist but wasn't really part of the "gang." He believed he was visited by aliens and eventually ran out into the street and was hit by a car. Another victim was the protagonist's friend, who began acting kind of weird and dreamy and was walking around naked at some point, because she either undressed herself or simply couldn't bother to put her clothes back on after phys-ed. She's hospitalized and survives, I believe. Towards the end the protagonist becomes "infected" as well. The supernatural force presents itself to her as a fairy feast of some kind.
They manage to somehow break the hold the supernatural thing has shortly after the protagonist is infected. I think the book ends with the protagonist lying outside and someone, I think her mom, checking if the mark/symbol is gone.