If left unchecked, a pandemic flu today could kill a total of 150 million people in 20 months.
Every pandemic flu, from that Spanish flu in 1918 to the 2009 H1N1 outbreak, has originated in birds.
So every year, flu researchers flock to Reed's Beach, in Delaware Bay in New Jersey.
There, a seasonal frenzy of migrating birds creates a unique breeding ground for the flu virus.
Taking samples from the birds' rear ends, and their feces, is critical ritual meant to help prevent the next pandemic flu.