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What is Blood phobia and explain in detail?
Blood phobia (also known as hemophobia or hematophobia in American English and haemophobia or haematophobia in British English) is an extreme irrational fear of blood, a type of specific phobia. Severe cases of this fear can cause physical reactions that are uncommon in most other fears, specifically vasovagal syncope (fainting).  Similar reactions can also occur with trypanophobia and traumatophobia.  For this reason, these phobias are categorized as blood-injection-injury phobia by the DSM-IV.  Some early texts refer to this category as "blood-injury-illness phobia."

Causes
Blood phobia is often caused by direct or vicarious trauma in childhood.  Though some have suggested a possible genetic link, a study of twins suggests that social learning and traumatic events, rather than genetics, is of greater significance.. Blood-injection-injury phobia (BII) affects about 4% of the population in the United States.The inclusion of BII within the category of specific or simple phobias in classificatory systems reflects a perception that fear has a primary role in the disorder. Consistent with this assumption, blood-injury phobia appears to share a common etiology with other phobias. Kendler, Neale, Kessler, Heath, and Eaves (1992) have argued from data comparing monozygotic with dizygotic twins that the genetic factor common to all phobias (agoraphobia, social phobia, and specific phobias), strongly predisposes a person to specific phobias.
The recognition of an inherited vulnerability common to all phobias is consistent with the notion that elevated trait anxiety predisposes one to anxiety disorders. Trait anxiety provides a background of affective arousal that permits a more rapid activation of the fight or flight response