Katherina K. Hauner, a postdoctoral fellow at the Northwestern University Feinberg School of Medicine, answers:

Under normal circumstances, fear triggers a natural fight-or-flight response that allows animals to react quickly to threats in their environment. Irrational and excessive fear, however, is typically a maladaptive response. In humans, an unwarranted, persistent fear of a certain situation or object, known as specific phobia, can cause overwhelming distress and interfere with daily life. Specific phobia is among the more prevalent anxiety disorders, affecting an estimated 9 percent of Americans within their lifetime. Common subtypes include fear of small animals, insects, flying, enclosed spaces, blood and needles.

For fear to escalate to irrational levels, a combination of genetic and environmental factors is very likely at play. Estimates of genetic contributions to specific phobia range from roughly 25 to 65 percent, although we do not know which genes have a leading part. No specific phobia gene has been identified, and it is highly unlikely that a single gene is responsible. Rather variants in several genes may predispose an individual to developing a number of psychological symptoms and disorders, including specific phobia.

As for the environmental component, a person may develop a phobia after a particularly frightening event, especially if he or she feels out of control. Even witnessing or hearing about a traumatic occurrence can contribute to its development. For instance, watching a devastating airplane crash on the news may trigger a fear of flying. That said, discerning the origin of the disorder can be difficult because people tend to do a poor job of identifying the source of their fears.

Our understanding of how and why phobias crop up remains limited, but we have made great strides in abating them. Exposure therapy, a form of cognitive-behavior therapy, is widely accepted as the most effective treatment for anxieties and phobias, and the vast majority of patients complete treatment within 10 sessions. During exposure therapy, a person engages with the particular fear to help diminish and ultimately overcome it over time. An individual might, for example, look at a photograph of the dreaded object or become immersed in the situation he or she loathes. Fortunately for those plagued by irrational fears, we can treat a phobia rapidly and successfully without necessarily knowing its origin.