(male narrator) On these humble pages, scientific inquiries stack one upon the next to form a magnificent castle of human knowledge. We're investigating how flu moves about the world and tracing back where viruses come from.

(narrator) In a new study, Trevor Bedford and his colleagues investigate the lifestyles and travel habits of flu to understand the origin of outbreaks. Think of it: when millions come down with the spring flu, someone existed in the world that had the flu, and now everyone with the flu descends from that one sneeze. You don't know that person, obviously. But we can establish their region.

(narrator) Trevor says, the person who sneezed the sneeze that gave rise to flu season...

(Trevor) That person was most likely, and usually, in South China, India, or Southeast Asia.

(narrator) You might be thinking, wow. That's crazy.

[chuckling] Yes, yes it is.

(narrator) Trevor compared the genetic material, the RNA, to flu samples from all over the world and put together a giant family tree of flu, which Trevor agreed to recreate using paper and pipe cleaners. This is my level of abstraction. Anyway, it's fine.

(narrator) Each circle is a flu sample. Here's the virus. Both share a common ancestor. This virus actually seeded two infections. Flu mutates, so mutation has occurred. Subsequent people infected.

(narrator) And this goes on and on. Now imagine almost 10,000 circles. That's the size of Trevor's tree, making it, perhaps, the largest study of flu evolution in history. Depending on how you parse it, I think this is it.

(narrator) What did the flu family tree reveal? That the life of flu has more twists than a pretzel factory. The biggest surprise is that influenza B behaves very differently from flu A.

(narrator) There are different kinds of flu. Flu A includes the strain H3N2, the most common and deadly strain of seasonal flu last year. H3N2, Trevor found, leads a very different lifestyle from flu B. So H3N2 evolves very quickly.

(narrator) So your immunity to H3N2 doesn't last long. The flu evolves, escaping our immune system. You'll get it every five, ten years of your life.

(narrator) Flu B lives a quieter life. It evolves more slowly. It infects a higher proportion of children who haven't been exposed to flu before. Everyone gets it as a kid. But then you'll have immunity to it for the next 20, 15 years.

(narrator) These flus also travel differently, Trevor found. Flu B can settle for years in one place.

(Trevor) One B variant remains in Southeast Asia for six years without spreading.

(narrator) H3N2 travels faster than small town gossip, which was kind of mysterious. Why would H3N2 travel, while flu B is stuck in one place? The answer--airplanes and the people who ride 'em. Adults travel around the world on airplanes and spread flu, whereas B infects children, and kids don't fly as much and don't spread the virus as quickly.

(narrator) Because flu B evolves more slowly and infects mostly kids, it travels when kids travel. And kids don't travel that much. H3N2 evolves more quickly, infects more adults, and rides with them all over the world. Trevor's data showed H3N2 often migrates from Southeast Asia, where flu circulates all year long. It heads to North America, where it deplanes, outcompetes local strains, and takes over. Knowing how different flus operate can help us make better vaccines. We could tailor B vaccines to specific regions. And H3N2's migration patterns tell us where to look for strains that dominate seasonal flu. All of this geographic work points you towards where should we be picking our vaccine strains from.

(narrator) Enigmas about this world are a dime a dozen. Funding to purchase and make this educational production But with every study published, accessible was provided by the U.S. Department of Education:

our view of the world gets a little more interesting. PH:1-800-USA-LEARN (V) or WEB:www.ed.gov. Until next time.