January’s cold, dry weather in many areas of the United States seems to usher in the perfect conditions for seasonal influenza — in humans. But for birds in Europe and Asia, flu season is already in full swing: An epidemic of the H5N8 flu has broken out among European poultry. In Asia, the H5N6 strain is widespread, and another strain, H7N9, has infected birds and even killed three people.

According to the Centers for Disease Control, flu activity in the United States is “elevated” as of the beginning of January, but there’s no epidemic. However, for those of us keeping an eye on flu headlines, questions remain, like: What do all those "Hs" and "Ns" in these flue names mean? And how do they relate to our seasonal flu shot?

As it turns out, there are many different kinds of influenza, and they typically mutate easily and frequently. The Hs and Ns in the names of some flu strains are short for hemagglutinin and neuraminidase — two proteins on the surface of the Type A influenza virus allowing it to grab onto and infect cells. Hemagglutinin has 18 subtypes, and neuraminidase has 11 — those subtypes are what the numbers refer to.

But that’s not all: Flu can be Type A, B, C or D. In humans, seasonal flu is caused by Type A and Type B. (Type A strains are more common, but Type B strains are thought to be more dangerous for children.) There’s also Type C, which is usually very mild, and Type D, which mainly infects cattle.

In the United States, the annual flu vaccine is based on strains that circulated in Asia the year before. This year, the flu vaccine protects against Type A strains H1N1 and H3N2, as well as the Type B “Brisbane” strain. (B strains aren’t divided into H and N subgroups, like Type A strains are — instead, they’re named for their place of origin.)

While many kinds of animals can get the flu, our main sources of infection — besides other people — are pigs and birds. Migrating birds carry avian flu all over the world, infecting domestic poultry flocks. The H5N8 epidemic in Europe has already led to the slaughter of hundreds of thousands of chickens, ducks and turkeys, and some 30 million birds have been killed in Korea to prevent the spread of H5N6.

While avian influenzas usually don’t infect humans, they can be deadly when they do. Experts think the flu strain that swept the globe in 1918 was a mutated bird flu. And the current H7N9 — a mild illness for ducks and chickens — has killed three people and sickened dozens in China.

So while most avian flus remain “for the birds,” public health officials are constantly alert — because there’s always the danger that the usually mild illnesses could mutate, spawning the next global outbreak.

This article is based on reporting by Helen Palmer that aired on PRI’s Living on Earth.