There are four types of influenza virus, labeled A, B, C, and D. But within those four types, the number of distinct subtypes, lineages, and strains runs into the hundreds, and new variants emerge every year. The two types responsible for seasonal flu epidemics in humans are influenza A and influenza B.
The Four Types of Influenza
Influenza viruses are divided into four broad types based on their core proteins:
- Influenza A is the most diverse and dangerous type. It infects humans, birds, pigs, horses, dogs, and other animals. Wild aquatic birds serve as the primary natural reservoir for most of its subtypes. Influenza A is the only type capable of causing pandemics.
- Influenza B circulates almost exclusively in humans. It causes seasonal epidemics alongside influenza A but does not have the same pandemic potential.
- Influenza C causes mild respiratory illness in humans and is not tracked as part of seasonal flu surveillance. It does not cause epidemics.
- Influenza D primarily affects cattle and has not been shown to cause illness in people, though lab studies suggest human infection may be theoretically possible.
Influenza A Subtypes: Where the Numbers Multiply
Influenza A is classified into subtypes based on two proteins on its surface: hemagglutinin (H) and neuraminidase (N). These are the “H” and “N” in names like H1N1 or H5N1. Scientists have identified 18 different hemagglutinin types and 11 different neuraminidase types. Because any H can pair with any N, there are potentially 198 subtype combinations.
Not all of those combinations circulate widely or infect humans. Most exist quietly in bird populations. Only a handful have caused sustained human illness. Right now, the two influenza A subtypes circulating in people during flu season are A(H1N1) and A(H3N2). The H1N1 strain currently in circulation descends from the virus that caused the 2009 pandemic, which is why it’s sometimes written as A(H1N1)pdm09.
Other subtypes occasionally jump from animals to humans. A(H5N1), the avian flu strain that has spread through poultry and dairy cattle in recent years, has caused 71 confirmed human cases in the United States since February 2024, with two deaths. No person-to-person spread has been documented, and the public health risk remains low, but these animal-origin subtypes are closely monitored because they carry the potential to spark a pandemic if they gain the ability to spread efficiently between people.
Influenza B Lineages
Influenza B does not have subtypes like influenza A. Instead, it split into two distinct genetic lineages in the 1980s: B/Victoria and B/Yamagata. For about two decades, both lineages co-circulated during flu seasons worldwide. Yamagata was the dominant lineage in 10 of those seasons, Victoria in six, and the two appeared in roughly equal numbers during four others.
That picture has recently simplified. Since around 2020, the Yamagata lineage has largely disappeared from global surveillance. Current seasonal flu vaccines are trivalent, meaning they contain three virus components: A(H1N1), A(H3N2), and B/Victoria. Yamagata is no longer included. So in practical terms, three distinct viruses drive flu season today.
How New Variants Keep Emerging
Even within a single subtype like A(H3N2), the virus is not static. Flu viruses change through two main processes that constantly generate new variants.
The first is antigenic drift: small, continuous mutations that accumulate as the virus copies itself. These gradual changes alter the virus’s surface proteins just enough that antibodies from a previous infection or vaccination may no longer recognize it well. Drift is the reason you need a new flu shot every year. Sometimes a single mutation in a critical spot on the surface protein is enough to make last year’s immunity less effective. As of early 2025, a particular subclade of A(H3N2) called J.2.4.1 has been rising rapidly in global circulation, illustrating how quickly drift can shift which version of the virus is dominant.
The second process is antigenic shift, which is far rarer and far more consequential. Shift happens when a flu virus from an animal population acquires a completely new surface protein, or combination of proteins, that most humans have never encountered. This can occur when viruses from different species mix genetic material, often in an animal like a pig that can be infected by both bird and human flu viruses. Shift is what creates entirely new subtypes capable of infecting millions of people with no pre-existing immunity. Every flu pandemic on record has been triggered by an antigenic shift in influenza A.
Why the Total Count Keeps Changing
Asking “how many flu viruses are there” is a bit like asking how many shades of blue exist. At the broadest level, four types. Within influenza A, 198 possible subtype combinations, though only a fraction have been found in nature. Within each subtype, an ever-growing catalog of genetically distinct strains and clades. Global surveillance networks sequence thousands of flu virus samples each year, and the naming system reflects that granularity: each isolate is labeled by type, host species, geographic origin, strain number, year of collection, and subtype.
For everyday purposes, what matters most is the small group causing human illness right now. Seasonal flu in humans is driven by three viruses: A(H1N1), A(H3N2), and B/Victoria. Those are the targets of the annual flu vaccine, and they are the strains your immune system is most likely to encounter. The vast reservoir of other subtypes in birds and animals is what keeps public health agencies on alert, because the next pandemic strain will almost certainly emerge from that pool.