I remember standing there as my preschool classmate, a pale-faced boy about 4 years old, walked up and started talking to my teacher. What he said was inaudible, but I’ve never been able to forget what happened next: He vomited all over a pint-sized wooden chair. It looked like melted cookies-and-cream ice cream. I never sat in one of those chairs in my classroom again.

Since that day in preschool, I’ve had a fear of vomit, a phobia clinically known as emetaphobia.

Ten percent of Americans have a phobia of some kind, says Steve Ilardi, associate professor of psychology. Being afraid of something is normal, but a phobia is an excessive and unjustified fear that interferes with a person’s ability to function socially.

People who have phobias tend to become anxious and fearful when they’re around a particular object or situation, or they try to avoid that object or situation completely, says David Holmes, professor of psychology. “These are real fears that really disrupt people’s lives,” he says.

My heart races whenever I see, hear or smell vomit, or if the arrival of vomit is imminent (as when someone is sick or extremely drunk). I get a feeling in my stomach like I’m on a roller coaster, and I tense up and break into a nervous sweat.

Patroiophobia: Fear of heredity

There are at least three different ways to develop a phobia, Ilardi says. In some cases, phobias develop spontaneously. There is also evidence to suggest phobias are related to genetics and can be inherited. Identical twins are more likely to share a phobia than paternal twins, and people who are adopted are more likely to share phobias with their biological parents than with their adoptive parents, Ilardi says.

Sophophobia: Fear of learning

In other cases, phobias can be “learned,” or picked up from a trusted individual who has a phobia. Ilardi once treated a patient who developed muriphobia — a fear of rodents — after seeing her mother jump on the family’s kitchen table when a mouse came in the room. From that moment on, he says, she was terrified of mice.

Phobophobia: Fear of fear

Phobias can also be classically conditioned if a stimulus is paired with a fear one or more times and the two become connected. Ordinarily it takes a number of trials to classically condition a response, but if the experience is very powerful, you can get what’s called one-trial conditioning, Holmes says.

When Shawn Lillig, Basehor senior, was 15 years old, he bought an amplifier for his band. The first time he stepped up to the microphone, he felt an electric shock in his teeth that knocked him to the ground. Now he uses a towel, book or broomstick to flip on metal light switches, and pulling chains to turn on light bulbs makes him woozy, he says. He also uses a wooden spoon to retrieve Pop Tarts from his toaster.

Trophobia: Fear of doctors

Phobias can be treated through a process known as graded exposure, which has a success rate of over 90 percent, Ilardi says. First, the most upsetting triggers or features of the object or situation are identified. Then, a psychologist can construct a hierarchy of stimuli, starting with something that is uncomfortable but not traumatizing. “You might put a snake in a box outside my door and slowly but surely, over the course of a number of trials, bring it in until I’m sitting there petting it,” he says.

Exposure-based treatment is not as bad as it sounds. “People are usually thinking you do like a Fear Factor and make them just overwhelmed with exposure, and it’s not that at all,” Ilardi says.

Whenever people complain of nausea or a stomach ache, I flee. When my roommate had the flu last year, I had to stay at a friend’s place. I also get really anxious around people who are drinking. I have a few friends familiar with my condition who lead me away when they see people getting sick or looking suspicious, but I think I might need to look into graded exposure.