A phobia is a type of anxiety disorder - and has been defined as : "an overwhelming and debilitating fear of an object, place, situation, feeling or animal." [Source UK NHS]

Some other definitions also add the word 'irrational'.

The causes of phobias - which are estimated to affect around 10% of people worldwide - have been debated for centuries, and are still far from clear. Although some phobias can have obvious direct links with traumatic events in the past, others - perhaps the majority - don't appear to have a straightforward explanation.

A number of phobias seem perfectly logical - though exaggerated. Examples would include fear of heights, rats, insects, snakes etc, which, as an inherited trait, would presumably give an evolutionary advantage. Family studies have shown that tendencies towards certain phobias can indeed be passed down generations.

It's currently thought that this is an 'epigenetic' (outside the genes) phenomenon. There is however, no known way for such fears to be passed along generations via DNA.

Other phobias seem to be entirely irrational - for example Trypophobiaplugin-autotooltip__plain plugin-autotooltip_bigTrypophobia

Trypophobia - a fear of (clusters of small) holes - was first formally described by Geoff G. Cole and Arnold J. Wilkins in their paper for Psychological Science August 2013 entitled

'Fear of Holes'

"The image most often reported as inducing the phobia is the seed head of the lotus flower. Other examples include soap bubbles and the holes in aerated chocolate." - fear clusters of small holes, or Bibliophobia - fear of books, or Heptadecaphobia - fear of the number 17.

Wikipedia maintains a list of recognised phobias here .