But there are four strains of flu that all cause the same symptoms, and they're different enough that they don't spread around the world the same way, scientists reported in the journal Nature on June 9.
The most common strain, H3N2, spreads more quickly than H1N1 (known as swine flu) and the B strains, the scientists found.
The H3N2 strain mutates rapidly, changing a key protein on its outside that helps it get around whatever immunity humans have developed - that's why you can get the flu every year, and why you have to get a new flu vaccine every year to protect yourself.
Because the strain evolves quickly, it's able to spread around the world quickly, too.
To see how it moves, scientists analyzed DNA from nearly 10,000 flu viruses of all four strains, and mapped where in the world they'd been found over 12 years. With all that data, they could see how quickly each strain spread, and what that means for the virus's evolution.
One data note: The maps look blank over Africa, Central America, the Middle East, and Russia because of a lack of data, not because flu doesn't infect people in those areas.
The map below shows where H3N2 strains came from in the 12 years of study (the colors indicate the region the viruses were found in), with bigger bubbles corresponding to more viruses:
Bedford et al.
The H3N2 strain is the most common form of the flu virus.
Previous research had shown that H3N2 viruses are always going around in East and Southeast Asia, and spread to the rest of the world in yearly flu epidemics - which is how we can predict what strains are coming each year. Between worldwide epidemics, H3N2 strains die out everywhere but the west. That means each yearly epidemic spreads out from that reservoir, including the new strains you need to get a fresh shot for.
Because of how quickly H3N2 mutates and spreads, it infects people of all ages, including adults who travel and spread it around the world.
In contrast, the H1N1 and B flu viruses don't evolve as quickly and tend to infect more children than adults. Since children don't travel as much as adults, these strains don't spread around the world as quickly as H3N2 does, the researchers said.
The map below shows where H1N1 strains came from, colors again indicate region, while bubble size indicates population infected:
Bedford et al.
The H1N1 flu virus doesn't spread as quickly to as many people as the H3N2 one does.
While the two maps look pretty similar, the significant difference is the size of the purple, light blue, and green bubbles in East and Southeast Asia. They're not as big for H1N1 as H3N2 - and the researchers think they know why.
They that the H1N1 and B viruses actually circulate continuously around the world between epidemics - the opposite of H3N2. So while a lot of H1N1 strains still come from Asia, the map shows they spread from other places too.
Though they don't mutate as quickly, when they stick around, the H1N1 and B influenza viruses are able to mutate and build up a lot of diversity.
Knowing more about how different strains of the flu spread might help scientists predict which strains are likely to become an epidemic - and how best to fight these epidemics.
"Ultimately, this means that we can look at the viruses circulating in Asia to get a good idea of which H3N2 virus might spread worldwide, but for H1N1 and B it's tremendously variable and the dominant variant can vary from one region of the world to another," senior author Colin Russell said in a statement from the University of Cambridge.
In other words, what works for making predictions about H3N2 won't work for the H1N1 and B strains. Armed with that knowledge, hopefully we'll be a little better prepared for future flu outbreaks.