Is the 21-day Ebola quarantine long enough?

(CNN) – From the beginning of the Ebola outbreak, the Centers for Disease and Prevention has insisted if someone gets Ebola, the symptoms will show up in 21 days or less. It is a cornerstone of the CDC's science.

And yet, the world health organization says not twenty-one...But forty-two days must pass before a country suffering an outbreak can be called Ebola-free. Why?

CNN's Tom Foreman reports.