An intense, persistent irrational fear of something that produces conscious avoidance of the feared subject, activity or situation is called a phobia. 

• Phobias can vary in degree and how much they interfere with healthy adaptation to the environment. Some otherwise normal and well-adjusted persons also have phobias. 

Phobias are mainly of three types : 

1. Specific phobias are those directed towards specific objects and situations and can be varied, e.g., acrophobia (fear of heights), pyrophobia (fear of fire), and hydrophobia (fear of water). 

2. Social phobia is a fear of social situations, and people with this phobia may avoid a wide range of situations in which they fear they will be exposed to, scrutinized and possibly humiliated by other people. 

3. Agoraphobia: is the term used when people developed a fear of entering unfamiliar situations. Social learning theories work on the principle that our experience be it positive or negative such as phobia of lizards/cockroaches are the result of learning process which start early in life. Small children can play with snakes; they are not aware of the danger involved. For them it is just another play object, as they grow up the fear of these things are instilled by their parents and society which is reinforced and accounts for reactions like phobia. A psychoanalytical account for the same could involve attribution to some unconscious > or/and repressed experiences. For example, suppose in your childhood you watched a group of roudy boys brutally torturing a cockroach/snake, which eventually died, although you going about the incidence after some days, but it might remain in back of your mind forever, which might explain your phobia to cockroaches which might remind you of the incidence and disturbs you emotionally.