This is a very unique fear, Triskaidekaphobia is the fear of the number 13, and it is more common than you think. People regard the number to be an unlucky number, almost ever since the days of Jesus Christ. At the last supper, Jesus was said to be the 13th guest. Maybe even beyond that. In my research I found this phobia probably also affected the vikings. The Norse God Loki was the 13th God, and may have been responsible for the murder of Baldr, who was the God of light and beauty. Loki was also the 13th guest to arrive at Baldr's funeral. Then of course there are modern superstitions about Friday the 13th. Did you know that any month that begins on a Sunday will have a Friday the 13th? It's true! I've seen it over and over again! And this past year, I believe I counted about 4 months with Friday the 13th--more than I've ever counted in a single year before!
Today, the fear still lingers. Many hotels have eliminated the 13th floor. Those that have more than 12 floors will jump from 12 to 14 because some guests will refuse to stay in rooms on a floor labeled "13". A lot of people refuse to buy houses with the number 13 in the address, or live in number 13 apartments. These people will even refuse to leave their houses on Friday the 13th. There is also a 13th constellation that astrologists refuse to discuss, called Ophiuchus. It is said to be a constellation of disaster. In 1970, Apollo 13 was launched at 13:13 hours, and the mission had to be aborted on April 13, because of an exploded oxygen tank. Coincidence? Maybe. Bad luck? I guess it depends on how you look at it. No one in the Apollo 13 mission was killed, they made it back home safe and sound. But I guess to someone with Triskaidekaphobia, it doesn't matter. About 50,000 people suffer from this phobia, and the 13 Club that meets every Friday the 13th is set out to prove this number is not as unlucky as it seems.
Associated with this phobia is other, similar phobias like Paraskevidekatriaphobia, which is the fear of Friday the 13th, and Hexakosioihexekontahexaphobia, which is the fear of the number 666. But if you suffer from Triskaidekaphobia, you probably don't want to see images like these:
The original 13-colony American flag.
Most ponies are usually 13-hands high.
The movie Thirteen (2003).
A 13-lined ground squirrel.
A 13-year old girl.
A 13-year old boy.