There are four types of influenza virus: A, B, C, and D. Only two of them, influenza A and B, cause the seasonal flu outbreaks that sicken millions of people each year. Influenza C causes mild illness, mostly in young children, and influenza D primarily infects cattle with no confirmed cases in humans.
But the story goes well beyond four types. Within influenza A alone, there are 18 subtypes of one surface protein and 11 of another, creating a huge number of possible combinations. Understanding these distinctions helps explain why the flu keeps coming back year after year and why some seasons hit harder than others.
Influenza A: The Most Dangerous Type
Influenza A is the type responsible for flu pandemics and the most severe seasonal outbreaks. It circulates in humans, birds, pigs, and other animals. What makes it uniquely dangerous is its ability to change dramatically. The virus carries two key proteins on its surface, labeled H (hemagglutinin) and N (neuraminidase), and these come in 18 and 11 varieties respectively. Each combination gets its own subtype name, like H1N1 or H3N2.
Only two subtypes currently circulate in people on a regular basis: A(H1N1) and A(H3N2). During the week ending January 11, 2025, U.S. surveillance data showed that about 57% of subtyped influenza A samples were H3N2 and 43% were H1N1, with a single H5 detection. H3N2 seasons tend to be rougher, particularly for older adults, though both subtypes can cause serious illness.
The real concern with influenza A is something called antigenic shift. Because the virus has a segmented genome (its genetic material is split into separate pieces), two different influenza A viruses infecting the same animal can swap entire gene segments. This can produce a radically new virus that the human immune system has never encountered. That’s the mechanism behind pandemics. The 2009 H1N1 pandemic, for example, emerged from a reassortment involving swine, avian, and human flu viruses. No other influenza type has this pandemic potential.
Influenza B: Equally Severe but More Stable
Influenza B circulates almost exclusively in humans. It doesn’t have the massive animal reservoir that influenza A does, which limits its ability to change dramatically. Instead of subtypes, influenza B split into two lineages in the 1970s: B/Victoria and B/Yamagata.
For decades, public health officials had to guess each year which B lineage would dominate, and a wrong guess meant reduced vaccine protection. That problem may have solved itself. The B/Yamagata lineage has not been detected in circulation since March 2020. Research published in Nature Communications found that COVID-era measures like masking and social distancing, combined with a large outbreak in 2017-2018 that left fewer susceptible people, drove B/Yamagata to probable extinction. Simulations suggest the lineage would have survived if it had undergone more genetic change around the time of the pandemic, or if public health interventions hadn’t been implemented. Based on this disappearance, the WHO recommended dropping B/Yamagata from vaccines starting with the 2024 flu season.
A common misconception is that influenza B is milder than influenza A. A CDC study examining over 24,000 flu-related hospitalizations across eight seasons found no significant difference in severity between the two types. Among hospitalized adults, influenza B caused comparable rates of ICU admission, similar lengths of hospital stay, and a similar proportion of deaths as influenza A. The key difference is reach, not severity: influenza A infects more people overall because of its greater genetic diversity and animal reservoirs.
Influenza C: Mild and Underrecognized
Influenza C is common but rarely makes the news. Most infections cause mild upper respiratory symptoms: fever, runny nose, and cough. It can occasionally lead to bronchitis or pneumonia, particularly in young children, who are more likely to be hospitalized from it. Gastrointestinal symptoms like nausea and vomiting are also frequently reported with influenza C, which is less typical of A and B infections.
Because the symptoms overlap with the common cold and routine testing doesn’t look for it, influenza C is significantly underdiagnosed. Most adults have been exposed by the time they reach adulthood and carry some immunity.
Influenza D: No Threat to Humans
Influenza D was discovered in pigs in Oklahoma in 2011 and has since been found in cattle, which appear to be its primary host. While cattle workers have tested positive for antibodies against the virus (suggesting some exposure), no confirmed productive infection in humans has been documented. Influenza D is not included in seasonal flu surveillance or vaccine planning.
Why the Flu Vaccine Changes Every Year
Even without dramatic genetic shifts, influenza A and B viruses constantly accumulate small mutations in the proteins your immune system recognizes. This process, called antigenic drift, is why immunity from last year’s flu (or last year’s vaccine) gradually becomes less effective. It’s also why the WHO convenes twice a year to select updated vaccine strains for the Northern and Southern Hemispheres.
All flu vaccines for the 2025-2026 season are trivalent, meaning they protect against three viruses: an A(H1N1) strain, an A(H3N2) strain, and a B/Victoria lineage strain. This is a shift from recent years, when quadrivalent vaccines included both B lineages. With B/Yamagata likely gone, the fourth component was dropped.
The specific strains selected also differ slightly depending on how the vaccine is manufactured. Egg-based vaccines use strains like A/Victoria/4897/2022 for H1N1 and A/Croatia/10136RV/2023 for H3N2, while cell-based and recombinant vaccines use A/Wisconsin/67/2022 and A/District of Columbia/27/2023 for the same subtypes. Both formats target B/Austria/1359417/2021 for the Victoria lineage component. These differences exist because viruses can mutate slightly when grown in eggs, so manufacturers choose strains optimized for each production method.
What Actually Circulates Each Season
In any given flu season, you’re dealing with a mix of influenza A(H1N1), A(H3N2), and B/Victoria. The ratio shifts from year to year and even week to week. Some seasons are dominated by H3N2, others by H1N1, and occasionally influenza B takes the lead. The severity of a flu season depends heavily on which subtype dominates, how much it has drifted from the vaccine strain, and which age groups are most vulnerable to that particular virus.
So while the textbook answer is four types of flu, the practical answer is more nuanced. Three viruses (two influenza A subtypes and one influenza B lineage) drive virtually all seasonal flu illness in humans, each one shifting just enough each year to keep your immune system guessing.