Hilary Brueck/Business Insider
A dangerous assassin lurks on the Jersey shore - a killer so deadly that it could wipe out 30 million people in one year.
That threat is a pandemic flu virus that scientists are not yet prepared to fight.
"The world needs to prepare for pandemics in the same serious way it prepares for war," Bill Gates said last year, on the 100-year anniversary of the Spanish flu, which wiped out 50 million people in 1918.
If left unchecked, a pandemic flu today could kill a total of 150 million people - near half the population of the US - in 20 months.
"If history has taught us anything, it's that there will be another deadly global pandemic," Gates said.
Read More: Bill Gates says a devastating 'quirk of nature' could kill 30 million people in a year. Researchers are fighting that threat by studying bird butts.
Every pandemic flu, from that Spanish flu in 1918 to the 2009 H1N1 outbreak, has originated in birds. They're nature's primordial hosts for pandemic flu.
So every spring, a troupe of flu researchers flock to the southernmost tip of New Jersey in Delaware Bay, where a seasonal frenzy of migrating birds creates a unique, perhaps even unparalleled, breeding ground for the flu virus.
Their annual research there, which involves taking samples from bird poop and butts, is a bizarre but critical ritual that scientists hope will help prevent the next pandemic flu before it's too late.
Take a look.