The textbook describes phobias as, "intense, irrational fears" and continues that many phobias such as fear of spiders, snakes, the dark, etc., are common place, most of the time without a frightening encounter. I myself have arachnophobia and the rest of my family doesn't understand why, if I've never or had a bad experience with a spider.
I compare this with my mother's fear of worms but she always defends herself with the argument that she's had a bad experience with worms when she was younger. I've heard this story multiple times: when she was in grade school her friends (young and foolish) decided to gross her out (psych terms- produce a reaction of disgust) and collected a bucket of worms on a rainy day. Mind you, previous to this, my mother had no problem with worms. Her friends rang the doorbell to her house and when she answered, they dumped the bucket of worms all over her head. My mother was absolutely horrified and since then has not been able to see or be near worms without getting extremely disgusted and uncomfortable. This argument of spiders versus worms often gets jokingly brought up and it was only after reading chapter 11 that I noticed one part of the story I had previously dismissed.
My mom always starts off with explaining in detail how she'd already had a bad day because my very strict grandparents had told her to clean her room and it didn't get done on time, which means she got in trouble. When her friends were at the front door, they rang the doorbell multiple times in a row and my mom was afraid she'd get into even more trouble and ran to get the door.
This brings me to the two factor theory of emotion, by Stanley Schachter and Jerome Singer. Perhaps the reason my mom developed an actual phobia of worms was because she experienced a state of fear (from the amygdala) and used a labeling process of fear to worms. However, a possible reason the reaction was so strong was because my mom was already in trouble and possibly could have had a rush of adrenaline when the doorbell was constantly rung, in fearful excitement of getting into even more trouble with serious consequences.
This possibility is backed by the famous Dutton and Aron experiment of 1974, and I think could possibly explain what happened with my mom. Due to the stress of previously being scolded mixed with the adrenaline and annoyance of getting in trouble again (when the doorbell was being constantly rung) her reaction to the worms being thrown on her was an extreme one, and produced more fear than if she hadn't been on the verge of getting in trouble again. However, nothing is concrete and this is just a possibility, which could be easily falsified. For example, many 'flashbulb memories' are remembered with elaborate details that actually didn't take place, and perhaps my mom is mistaken, which is a possibility, along with the possibility that all of the theories of emotion hold some small truth.
To end with, I think phobias are incredibly interesting and often give an insight to the person in question. There are many phobias in the world, and plenty of them are not irrational!