From Wikipedia 

Phobias 01

A phobia (from the Greek: φόβος, phóbos, meaning "fear" or "morbid fear") is an intense and persistent fear of certain situations, activities, things, animals, or people. The main symptom of this disorder is the excessive and unreasonable desire to avoid the feared stimulus. When the fear is beyond one's control, and if the fear is interfering with daily life, then a diagnosis under one of the anxiety disorders can be made.

This is caused by what are called, neutral, unconditioned, and conditioned stimuli, which trigger either conditioned or unconditioned responses. An example would be a person who was attacked by a dog (the unconditioned stimulus) would respond with an unconditioned response. When this happens, the unconditioned stimulus of them being attacked by the dog would become conditioned, and to this now conditioned stimulus, they would develop a conditioned response. If the occurance had enough of an impact on this certain person then they would develop a fear of that dog, or in some cases, an irrational fear of all dogs.

Phobias are the most common form of anxiety disorders. An American study by the National Institute of Mental Health (NIMH) found that between 8.7% and 18.1% of Americans suffer from phobias. Broken down by age and gender, the study found that phobias were the most common mental illness among women in all age groups and the second most common illness among men older than 25.

Phobias are not generally diagnosed if they are not particularly distressing to the patient and if they are not frequently encountered. If a phobia is defined as "impairing to the individual", then it will be treated after being measured in context by the degree of severity. A large percent of the American population is afraid of public speaking, which could range from mild uncomfortability, to an intense anxiety that inhibits all social involvement.

Phobias are generally caused by an event recorded by the amygdala and hippocampus and labeled as deadly or dangerous; thus whenever a specific situation is approached again the body reacts as if the event were happening repeatedly afterward. Treatment comes in some way or another as a replacing of the memory and reaction to the previous event perceived as deadly with something more realistic and based more rationally. In reality most phobias are irrational, in the sense that they are thought to be dangerous, but in reality are not threatening to survival in any way.

Some phobias are generated from the observation of a parent's or sibling's reaction. The observer then can take in the information and generate a fear of whatever they experienced.