What does the Pararibulitis exactly do to the concerned person?
Trying to write an oversimplified answer if you want the details of how people would experience such a thing.
In a room, there is a light switch, which is connected to a light bulb. Somewhat obviously, turning the switch on will send electricity to the light bulb, causing it to turn on.
Imagine if I hacked into the wiring. I connected another cable to the system. I send electricity to the bulb using this new cable. Now, the light bulb is on even though the switch in the room is still off.
The human brain works the same way. Your nerves send an electrical signal to your brain. Let's say you crush your hand under something heavy. All the nerves in that region will send an electrical signal to the brain, and the brain will receive many "distress calls".
It's important to note that this electrical signal does not "contain" pain. It is simply a message. The brain, upon reading this message, will create the experience of pain in order to alert the person that their body is hurt.
Similarly, a letter does not carry literal tears or heartache. But if the letter tells the recipient that their loved one has died, they will still feel sad and hurt. Because the recipient interprets the message as sad.
And now we get to the pararibulitis. It has basically hacked the electrical wiring of a person's body. It makes it possible for an electrical signal to be sent to the brain without the nerve having actually sent a message.
In this case, the person's brain creates the experience of pain (the light bulb turns on) because it thinks the nerve (light switch) has sent a distress signal that the body is hurt.
But in reality, the pararibulitis (the electrical wire that was added) has sent the message; and the brain is not capable of recognizing this (just like the light bulb does not know where the electricity is coming from).
If I tell humanity that aliens are definitely invading Earth, there will be panic.
But suppose I lied about that. I have no proof. But I still claim that the aliens are coming. And humanity believes what I say, because they have no way of knowing that I'm lying.
The panic will be created regardless of whether aliens are invading or not. The panic is created because humanity thinks that it is happening. It doesn't need to be true, it just needs to be believable.
Just like how Amanda's brain thinks she is drowning (and therefore her body panics) even though there's actually no water in her lungs.
I hope the explanation was helpful.