Spoilers ahead for the Riverdale Season 3 midseason finale. The seizures on Riverdale started out small. First Betty, then Ethel, then Josie. But in the midseason finale of Season 3, the whole Vixen team falls prey to whatever is causing the seizures on Riverdale. Well, all of them except for Cheryl, who's left screaming in horror when her team drops to the ground out of nowhere.
There may be a very specific reason that Cheryl was the only Vixen not affected, though. It seems that the Blossoms are in on whatever shady thing is going on with Hiram Lodge. If Cheryl's mom and/or uncle is part of the dealings, they may have helped Cheryl stay safe from whatever is poisoning the other Riverdale high school girls.
But what is poisoning them? There are a couple of different theories floating around. One theory is that Hiram Lodge is poisoning the Sweetwater River water source with Fizzle Rocks in an effort to brainwash everyone with Gargoyle King hallucinations and/or get the citizens addicted to sell even more of the drug.
It's notable that Veronica didn't have any seizures until after she moved out of her parents' home. If Hiram is tampering with the Riverdale water, he would certainly ensure that his own house was safe. Veronica became vulnerable when she left and had a seizure shortly afterward, which forced her to move back home.
The other theory is that the poison is coming from the Blossom maple syrup, since Claudius Blossom seems to be involved. According to medical journal Epilepsy Currents, one cause of seizures is having abnormal glucose (sugar) levels. Both the Fizzle Rocks candy and maple syrup have sugar in them. Maybe Hiram and/or Claudius spiked those levels in their products to induce seizures in the town.
Also, it's possible that the poison is oleander, which can cause seizures in some cases, as Reddit user Witchazel9 discovered. Oleander is what killed Dilton Doiley and it's been alleged that the Blossoms grow it. Maybe that's where the poison is coming from.
In any case, because even people who aren't playing Griffins and Gargoyles are experiencing seizures, it must be something bigger than the game that's hurting them, as Redditor dcfb2360 noted. The user added that maybe it wasn't Hiram who poisoned the water, but the Farm cult á la Jonestown. It does seem like Riverdale is trying to point the finger at Edgar Evernever's daughter, since she was present and acting oddly during several of the seizures. But that could be a classic Riverdale misdirection.
It's also possible that not every seizure has been connected to the same source. Betty had hers so early on in the season, and it came right after she confessed to abusing amphetamines. So hers could have been a side effect of medicine overuse that inspired whoever the Gargoyle King is to give the rest of the town seizures, too.
Whatever the cause, the outbreak got the town quarantined. What Hiram and co. have planned next is anyone's guess, but it's likely we haven't seen the last of the seizures — and the trend shows that they're probably only going to get worse and affect more people as time goes on.