Millions of people in Iraq for Arbaeen

It is one of the largest religious gatherings in the world. Arbaeen remembers the death of Imam Hussein back in the 7th century. He was the grandson of the Prophet Muhammad. There are more than 21 million people estimated to be in Iraq right now, walking toward the city of Karbala. The World's Sara Hassan reported on the event from Iraq during last year's Arbaeen.