History.com: What's So Unlucky About the Number 13?

Unexplained fears surrounding the number 13 can be traced to ancient times. Researchers estimate that as many as 10 percent of the U.S. population has a fear of the number 13, and each year the even more specific fear of Friday the 13th, known as paraskevidekatriaphobia, results in financial losses in excess of $800 million annually, as people avoid marrying, traveling or in the most severe cases, even working. But what's so unlucky about the number 13, and how did this numerical superstition get started?