Two stories to bolster the "like an allergic response" analogy:
One of my coworkers is afraid, no, *phobic* of bugs. Mostly the flying or the crawling varieties (I think worms are not a concern?).
She has worked on this phobia because she doesn't like that going outside is fraught with anxiety and terror of seeing or interacting with a bug.
So she has done whatever she's done to make going outside (where bugs are) more tolerable.
I was in the bathroom stall at work, there are two stalls. I was finishing up and going to flush.
Someone comes into the bathroom and enters the other stall.
I don't make it a habit to find out who exactly just came into the bathroom, so the other stall was simply now Occupied and I was just about to leave my stall.
While turning to face the stall door to leave, movement caught my eye on the floor between the stall dividers. It was a bug crawling its way across the floor, somewhat into the other stall.
Oh, ok, cool, did not accidentally step on a bug, yay.
I found out who was in the other stall when suddenly there was a scream and the sound of mad-scrabbling in the stall.
You see, my coworker had worked very hard to desensitize herself to bugs. Outside. Where bugs live.
But this was a Surprise Bug.
This bug was not expected and therefore my coworker had not preemptively girded her loins against to possibility of coming across a bug.
In fact, my coworker was in a very vulnerable and private space, a space that Should Not Have Had A Bug.
My coworker busted out of her stall, pants mid-thigh, and slammed into the sink counter.
Her reaction completely bypassed her brain. This was a mindless run-run-run instinct based on terror. She was shaking and gulping air, and had her pants mid-thigh.
Once she had distance from the bug, and she was able to begin coming back online mentally, she was beyond embarrassed.
She knew her reaction was massive overkill, and she had a witness, and she was partially undressed in front of said witness.
She knew on every level that this reaction was unhinged.
That did not change the fact that she, when faced with her fear - in an unexpected place - at a vulnerable time, reacted like someone had just tried to attack her.
She's done work on this, she knows cognitively that bugs exist and they are generally Not A Problem.
But like an allergic reaction, her body/brain completely misfires and reacts like this Non-Entity is equivalent to her Actively Being Lit On Fire.
I tried to be as non-reactive as possible, and check-in on her, but ultimately she needed a moment to breathe and collect herself.
I washed my hands, got the bug, and left.
We briefly spoke later, she apologized, I said thank you but unnecessary. She clearly was in the middle of a crisis at that moment. It was pure hind-brain-terror in control.
She is apparently working more on her phobia because her reaction, and that it happened "publicly", made her realize how severe her phobia is (and she would like to never have that happen again).
One of my parents was afraid of spiders (arachnophobia). Other insects/bugs/etc were fine. But not spiders.
Their reaction was to scream & jump away from the spider, generally trying to get as far away as quickly as possible. And they would not go back into the space until the spider had been removed
(they preferred if the spider was killed and the corpse removed [this part of their phobia was unacceptable - you do not get the right to kill something just because you are terrified of it], we just ushered the spider elsewhere).
While inconvenient, their phobia was not generally life-altering.
Usually spiders stay away from people, or they blend in and are unnoticeable.
But, sometimes, you get Surprise Spiders.
Like when your arachnophobia parent is driving, and a spider comes up from who knows where, and is crawling across the dashboard.
My parent screamed, jerked the wheel, and nearly drove us into a tree. Fortunately they also happened to slam the brake (while their other foot was still on the gas, it was...not great for the car). They had enough presence of mind to shift into park before scrambling out the door.
We were so so lucky. The street was empty besides us. There were no pedestrians around. And we did not hit anything besides the curb.
This is not a rational, cognitive reaction.
Arguably it was antithetical to survival.
They did not work on their phobia at all because it mostly sorted itself. So when their phobia came up unexpectedly, they were driven purely by instinct and had not built up a fall-back of "try to breathe, try to give yourself mental distance from the gibbering terror in your mind, use these mental/emotional tools to try to regulate your reaction so you don't fucking kill yourself or the people around you"
These two instances are, hopefully, illustrating that phobias aren't logic driven. They aren't rational or cognitive. If you ask someone why they have such a fear of X thing, they may provide an explanation. But I've found the explanation is usually an attempt to rationalize a completely instinct-based emotional response - a response that often feels like it hijacks your brain.
Which is not to say people with phobias can't / shouldn't work on them. They absolutely can / should, if possible.
From a completely selfish perspective: I would rather not be in a situation again where I'm almost killed because someone unexpectedly came across their phobia.
But I also recognize that in both the above instances, they were jump-scared, and they reacted with a knee-jerk response.
As teaboot said, phobias are like an allergic response.
The great thing about them though, is that they *aren't* your immune system.
They happen in the brain / nervous system, and so you have a modicum of control and agency (unlike an overactive immune system that basically tells you - go fuck yourself, it's having a moment - and all the deep breathing and rationalizing and other self-regulating tricks won't make your face and throat un-swell).
You can work toward desensitizing yourself or giving yourself tools to regulate when you come across your phobia.
And people who don't have phobias (or you haven't come across the thing that is your as-of-yet-unknown phobia): someone reacting to their phobia isn't going to be helped by you saying 'that? you're afraid of that? but that's just a X. it's not scary or dangerous' [that's like saying to someone who's drowning "just kick your legs and push your arms through the water"].
Yeah, no shit. The reaction isn't a well-reasoned, evidence-based parry to a sudden stimuli. It's an emotional immune system on fire.
It's more helpful to create space (physical distance from The Thing), removing The Thing (if possible), and trying to assist the person having the reaction (either by helping them leave, or possibly getting in between them and The Thing, if they have the brainspace trying to help them focus any de-escalation tricks they might have).
