The setting is modern Earth, and you're all normal people. However, you're normal people with a twist - you've contracted a psychological disease: Cyclic Psychoactive Disorder. CPD is characterized by periodic (hence the cyclic) episodes of hallucinations. There are also 'triggers', things you see or hear such as abstract art or music, though it varies based on person, which cause you to have an episode. These episodes can last mere minutes to entire days. Even more disturbing, if one infected person has an episode, this can cause other infected persons in the area to also have the same hallucination. This is a phenomenon called "syncing up." They are mass hallucinations.
CPD is not recognized by the American Mental Association as a psychological disease. This has caused a number of underground support groups to spring up. The support groups assign each member a sponsor whom you are supposed to call after you come out of an episode. Never call the sponsor if you suspect you are having an episode, however, as this can cause them to have an episode too. The support group also advises not to draw attention to oneself, and especially to stay away from the authorities. It is emphasized by the support groups that the hallucinations are not real. Do not embrace the hallucination. Do not treat it as real or express curiosity about the hallucination. Do not open doors that you don't remember being there.
Especially do not follow any white rabbits down holes.
"We have a new member in our group," said the support group leader, Adam Light. "Please introduce yourself."
A man stood up, shaking a bit. "My name is John, and I'm infected." There was a slight tsk from Adam, but he said nothing. John cleared his throat.
"I became infected last week, I think. At least, that's when I first had the hallucinations. Like just yesterday, I woke up and went to work - and everything seemed a bit...off. People were acting strangely."
"What caused you to be infected?" Adam said in a tone that suggested he was expecting a certain answer.
"I, saw a door that I hadn't seen before, and I was curious so I opened it."
"And that, my friends, was a Wrong Thing," Adam said, shaking his head slightly. "But you didn't know better. Now you do - do not open doors out of curiosity. Do not believe Impossible Things, either. This is how you become Infected." He put an emphasis on 'Infected' that suggested this was the proper way to pronounce it. "Now go on, explain your hallucinations."
"People acted crazy. Our intern poured coffee in my plants and gave me plant food. My boss sat on his desk instead of his chair. And if this wasn't odd enough, when I finally snapped out of it, everyone claimed that I was the one who was acting odd! They said my eyes were constantly wandering when being talked to, and that what I said didn't really make any sense. It was grammatically correct, and vaguely matched the conversation, but it was just wrong. This is exactly what I thought was happening with them! And then they said that at around 2 I went into the copy room and made copies of, er, my rear end. Well I did go into the copy room around 2, but I swear I was making copies of the fourth quarter financial report. Yet when I stopped hallucinating, they showed me pictures of my bum! I can't deny that, now can I?"
"Thank you John. Remember, these hallucinations make you act crazy while thinking everyone else is crazy. Does anyone have anything to say to John?"
A young woman, about college age, raised her hand.
"Hello John, I'm Ashley. Remember, your hallucinations are not real. They are in your head."
"Thank you, Ashley," Adam said. "Anyone else?"
"Oh, John, when you're having your hallucinations, try to stay away from the authorities," a man with messy brown hair said.
"Thank you, David. Yes, since you'll be acting strange they might try to question you, and if you're acting too strange you could find yourself arrested. And what's worse is that you might hallucinate something entirely different and wake up in jail!"
"I have something to say," another man said, looking around nervously. "Where did you put my cheese? Come on now, I see that you're all rats, so you must have the cheese! I know you're rats because you could fit into the rat holes, yessirree. Give my cheese back, you stole it!"
"Oh crap!" Adam said, standing up and rushing over to this man. "Session dismissed, everyone get out of here immediately!"