There are four types of influenza virus: A, B, C, and D. Only two of them, A and B, cause the seasonal flu that sends millions of people to bed each winter. The other two are either too mild to notice (C) or don’t infect humans at all (D). But within those four types, the variety is enormous, with more than 130 subtypes of influenza A alone identified in nature.
Influenza A: The Most Dangerous Type
Influenza A is the type responsible for every flu pandemic in recorded history. It circulates widely in humans, but it also infects birds, pigs, horses, whales, and seals. That broad host range is exactly what makes it so dangerous. When a version of the virus jumps from animals to people, it can trigger a global outbreak because human immune systems have never encountered it before.
Influenza A viruses are classified by two proteins on their surface: hemagglutinin (H) and neuraminidase (N). These are the “H” and “N” in names like H1N1 or H3N2. Scientists have identified more than 130 combinations of these proteins in nature, mostly in wild birds, and the actual number of possible combinations is likely higher because flu viruses can swap genetic material with each other in a process called reassortment.
Despite all those subtypes circulating in animals, only a few regularly infect humans. The subtypes in seasonal circulation right now are H1N1 and H3N2. These are the strains included in each year’s flu vaccine, alongside influenza B.
Why Influenza A Changes So Fast
Flu viruses evolve through two mechanisms. The first, called antigenic drift, is a slow, steady accumulation of small mutations. This is why you need a new flu shot every year: the virus drifts enough from one season to the next that last year’s immune protection no longer fits well.
The second mechanism, antigenic shift, is far more dramatic. It happens when two different influenza A viruses infect the same animal (often a pig) and exchange large segments of their genetic code, producing a virus with a completely new surface protein. If that new virus can spread efficiently between people, the result is a pandemic. Antigenic shift only occurs in influenza A because it’s the only type with such a wide range of animal hosts and subtypes available to recombine.
Influenza B: Seasonal but Less Variable
Influenza B causes seasonal flu alongside influenza A, and it can make you just as miserable during any given illness. The key difference is scope. Influenza B circulates almost exclusively in humans, so it doesn’t have the massive animal reservoir that gives influenza A its pandemic potential. It also doesn’t have subtypes. Instead, it’s divided into two lineages: Victoria and Yamagata.
For years, seasonal flu vaccines included both lineages. That changed recently. No Yamagata lineage viruses have been detected anywhere in the world since March 2020, marking five consecutive seasons without a single confirmed case. Because of this disappearance, the U.S. FDA recommended switching from a four-strain vaccine to a three-strain vaccine for the 2024-25 and 2025-26 seasons, dropping Yamagata entirely. The current vaccine includes one H1N1 strain, one H3N2 strain, and one B/Victoria strain.
Whether Yamagata is truly extinct or simply circulating at undetectable levels remains an open question, but for now, Victoria is the only influenza B lineage in play.
Influenza C: Mild and Common
Influenza C infects humans but generally causes mild respiratory illness, more like a common cold than what most people think of as “the flu.” It doesn’t cause seasonal epidemics and isn’t tracked the way A and B are. Most people are exposed to influenza C during childhood without ever realizing it. It’s not included in flu vaccines because it simply doesn’t pose enough of a public health threat.
Influenza D: A Livestock Virus
Influenza D primarily affects cattle, with occasional spillover to pigs and other animals. It is not known to cause illness in people. Serological studies (blood tests looking for antibodies) suggest that people who work closely with livestock may occasionally be exposed, but no confirmed human illness from influenza D has been documented. In one surveillance study, viral genetic material was found in the nasal passages of a farm worker in Asia, though researchers noted this could reflect passive nasal carriage rather than an actual infection.
What This Means for Flu Season
Of the four types, only A and B matter for your annual flu shot and your risk each winter. Influenza A is the wildcard, capable of producing new subtypes that catch the global population off guard. Influenza B is more predictable but still contributes to tens of thousands of hospitalizations each year. The seasonal vaccine is reformulated annually to match whichever specific strains of A/H1N1, A/H3N2, and B/Victoria are expected to dominate.
The sheer number of influenza A subtypes circulating in birds and other animals is also why public health agencies monitor avian flu outbreaks so closely. A subtype like H5N1, which currently spreads among poultry and some mammals, doesn’t yet transmit easily between people. But influenza A’s ability to reassort means that could change, and that possibility is what keeps global surveillance systems running year-round.