Every year brings a new strain of flu. Here’s how it all starts.

We tend to take seasonal flu's unwelcome arrival every fall for granted. But where do these viruses come from? Our new video explains:

The video is based on a new study in the journal Nature that sheds light on the answer. The researchers report that the flu has surprisingly predictable travel patterns — as long as you know which strain is the culprit.

The new research is important because knowing how flu travels can help us make more effective vaccines.

A massive flu family tree

Researchers Trevor Bedford, Colin Russell, and others put together an enormous family tree of different flu strains to try to understand how flu evolves and travels. Since the flu virus mutates quickly, the tree was designed to trace the ancestry of modern flus.

Bedford's flu family tree.

There are four primary strains of seasonal flu: H3N2 (the most common and deadly), H1N1, and two strains of influenza B. These strains, Bedford and Russell found, travel really differently. Flu B and H1N1 will often circulate for years in one area. H3N2, however, is more of a jet-setter: it travels quickly across the globe.

Our H3N2 seasonal flu strain almost always comes from East and Southeast Asia, where flu circulates all year long, Bedford says. In the fall, people sick with the flu board airplanes, travel to North America, and seed infections in the US. "This will start a chain of infection that will ramp up and constitute the flu season," Bedford says. This strain will die out in the US around March, and the cycle will repeat again the following season. "The only flus that matter in the evolutionary sweepstakes are these viruses that are in East and Southeast Asia," Bedford says.

Flu B and H1N1, on the other hand, can persist for a few years, but stay relatively local. Their limited range seems to be related to the fact that these strains evolve slower and infect more kids than adults. And kids don't travel as frequently.

If researchers understand the flu better, they can build better vaccines

The current process for building the seasonal flu vaccine resembles an educated guessing game. Researchers try to predict which of the circulating flus will spread, and build an injection that inoculates against those particular strains. Sometimes this can result in a mismatch, and a vaccine that isn't as effective as it could be.

The flu vaccine is usually only 40 to 60 percent effective in preventing the disease, Bedford says. And last year, it was an especially bad mismatch: the flu vaccine only reduced the odds of catching the flu by 25 percent.

If we know what region flu strains typically migrate from — the type of information the Nature article contains — we can focus on that region for selecting virus strains to put in vaccines.

"Seasonal flu kills 30,000 people in the US each year," Bedford says. "If we understood this bit of evolution better, we could make better vaccines."

Watch the video above to learn more. This video was made possible by the Alfred P. Sloan Foundation. Never want to miss a new Vox Video? Subscribe.

Related: Why you should get the flu shot every year