There are four types of influenza virus, labeled A through D. Only two of them, influenza A and influenza B, cause the seasonal flu that sends millions of people to bed each winter. Influenza C causes mild illness that most people barely notice, and influenza D primarily infects cattle and isn’t known to make humans sick at all. Understanding what separates these types helps explain why flu season looks different every year and why the vaccine changes annually.
Influenza A: The Most Dangerous Type
Influenza A is the heavyweight of the flu world. It’s the only type capable of causing pandemics, those global waves of illness that sweep across continents. The reason comes down to biology: influenza A infects a wide range of hosts, including birds, pigs, horses, and humans, giving the virus more opportunities to mutate and produce entirely new strains that human immune systems have never encountered.
The virus is classified into subtypes based on two proteins that stud its surface. There are 18 known versions of one protein and 11 of the other, creating a huge number of possible combinations. That’s where names like H1N1 and H3N2 come from. Only two subtypes currently circulate widely in people: A(H1N1) and A(H3N2). Wild aquatic birds serve as the primary natural reservoir for most of the other subtypes, which is why you hear about “bird flu” in the news. These animal strains don’t easily jump to humans, and none of the currently circulating zoonotic strains have achieved sustained person-to-person transmission. But the potential is always there.
Influenza A also mutates faster than the other types, which is a big part of why flu vaccines need to be updated every year. The 2025-2026 U.S. flu vaccines include an A(H1N1) component and an A(H3N2) component, both matched to recently circulating strains.
Influenza B: Seasonal but Not Mild
Influenza B is sometimes dismissed as the “milder” flu, but that reputation is misleading. A CDC study comparing outcomes among hospitalized adults found that influenza B caused equally severe illness as influenza A. Length of hospital stay, ICU admission rates, and the proportion of deaths were all comparable between the two types. If you’re sick enough to be hospitalized, the letter on your diagnosis doesn’t predict how bad it will be.
Where influenza B genuinely differs is in its scope. It only infects humans (and occasionally seals), so it doesn’t have the animal reservoirs that give influenza A its pandemic potential. It also evolves more slowly, meaning your immune system’s memory of a past B infection stays useful for longer.
Instead of subtypes, influenza B is divided into two lineages: B/Victoria and B/Yamagata. For years, both circulated side by side, which is why vaccine makers developed four-component (quadrivalent) vaccines to cover both. That picture changed dramatically during the COVID-19 pandemic. B/Yamagata hasn’t been confirmed in circulation since March 2020, and every lineage-identified type B virus detected since then has been B/Victoria. Scientists are now investigating whether B/Yamagata has gone extinct entirely. As a result, the 2025-2026 flu vaccines have shifted to a three-component (trivalent) formula that includes only B/Victoria.
Influenza C and D
Influenza C infects humans but generally causes only mild respiratory symptoms, more like a common cold than a true flu. It doesn’t cause epidemics and isn’t tracked the way A and B are. Most people have been exposed to it by adulthood without ever knowing it.
Influenza D was identified more recently and primarily affects cattle, with occasional spillover to pigs and other livestock. It has not been shown to infect or cause illness in people.
How Flu Viruses Change Over Time
One of the reasons flu keeps coming back is that influenza viruses are constantly shape-shifting. This happens through two distinct processes.
The first is called antigenic drift. Small, random mutations gradually alter the virus’s surface proteins, the parts your immune system learned to recognize from a previous infection or vaccination. These changes accumulate over time, which is why you can catch the flu more than once in your life and why health authorities review and update vaccine formulas for both the Northern and Southern Hemispheres every year.
The second process, antigenic shift, is far more dramatic and only happens with influenza A. This occurs when the virus undergoes an abrupt, major change, essentially swapping out its surface proteins for entirely new ones. This can happen when a flu virus from an animal population gains the ability to infect humans. Because most people have little or no immunity to the new combination, the result can be a pandemic. The 2009 H1N1 pandemic is a clear example: an H1N1 virus emerged with genes from North American swine, Eurasian swine, human, and bird influenza viruses, spread rapidly, and infected people worldwide.
How Flu Spreads and Survives
All types of influenza spread primarily through respiratory droplets produced when an infected person coughs, sneezes, or talks. The virus can also survive on surfaces for several hours to days, depending on the material. Hard, nonporous surfaces like doorknobs and countertops tend to keep certain viruses infectious longer, while fabrics may favor others. This is why hand hygiene matters during flu season, not just avoiding coughs.
What This Means for Vaccination
Because influenza A and B are the only types that cause significant illness in humans, seasonal flu vaccines target both. The current 2025-2026 U.S. vaccines are trivalent, containing three components: one A(H1N1) strain, one A(H3N2) strain, and one B/Victoria strain. The shift from four components to three reflects the apparent disappearance of the B/Yamagata lineage.
The yearly reformulation exists because of antigenic drift. Even small mutations can make last year’s vaccine a poor match for this year’s circulating strains. Getting vaccinated each season gives your immune system an updated blueprint of what the virus looks like right now, rather than relying on outdated information from a previous year’s shot or infection.