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 people to bed each winter. But within those types, dozens of subtypes and lineages circulate, mutate, and swap genetic material, which is why “the flu” feels like a different illness every year. Globally, these viruses cause around a billion infections annually, including 3 to 5 million severe cases and an estimated 290,000 to 650,000 respiratory deaths.
The Four Types of Influenza
Influenza viruses are classified into four broad types, each labeled with a letter.
- Influenza A is the most dangerous and versatile. It infects humans, birds, pigs, and other animals, and it’s the only type capable of triggering pandemics. The subtypes currently circulating in people are H1N1 and H3N2.
- Influenza B also drives seasonal epidemics alongside type A, but it circulates almost exclusively in humans and doesn’t produce pandemics.
- Influenza C infects people but generally causes only mild respiratory illness. It doesn’t spark epidemics.
- Influenza D primarily affects cattle and can spill over into other animals, but it is not known to cause illness in people.
When people talk about “flu season,” they’re really talking about influenza A and B trading dominance from one winter to the next.
How Influenza A Subtypes Work
Influenza A is subdivided by two proteins on the virus’s surface: hemagglutinin (H) and neuraminidase (N). These proteins are what your immune system recognizes when it tries to fight off infection. Different combinations of H and N create different subtypes, labeled with numbers like H1N1 or H3N2.
Many H and N combinations exist in nature, especially among wild birds. Only a few subtypes, however, routinely infect humans. Right now, the two circulating human subtypes are H1N1 (the strain behind the 2009 pandemic, still circulating in a seasonal form) and H3N2. The current U.S. flu vaccine is trivalent, meaning it targets three viruses: one H1N1 strain, one H3N2 strain, and one influenza B strain.
Influenza B and the Disappearing Lineage
Influenza B has historically split into two lineages: Victoria and Yamagata. For years, vaccine manufacturers included both in a four-component (quadrivalent) vaccine because either lineage could dominate a given season. That picture changed after the COVID-19 pandemic. A systematic review published in The Lancet Microbe found that B/Yamagata has been virtually undetectable since March 2020 and is likely extinct. The handful of cases reported since then were mostly traced to vaccine-derived virus or data entry errors.
Because of this, regulators have shifted flu vaccines back to a trivalent formula covering H1N1, H3N2, and B/Victoria only. If Yamagata truly is gone, seasonal flu lost one of its four major players.
Why the Flu Changes Every Year
Flu viruses evolve through two distinct mechanisms, and understanding them explains why you can catch the flu more than once.
The first is called antigenic drift. As flu viruses copy themselves inside your cells, small mutations accumulate in those surface proteins. Over months and years, these tiny changes add up until your immune system no longer recognizes the virus from a previous infection or vaccination. Drift is the reason vaccine formulas are reviewed and updated every year for both the Northern and Southern Hemispheres.
The second mechanism, antigenic shift, is rarer and more dramatic. It happens when an influenza A virus undergoes an abrupt, major change, typically by swapping genetic material with a flu virus from another species. The result can be a subtype that most humans have never encountered, leaving populations with little or no immunity. Antigenic shift is the engine behind pandemics. Drift is constant background noise; shift is the occasional earthquake.
Bird Flu and the Risk of New Strains
Wild birds are the natural reservoir for a wide range of influenza A subtypes, and occasionally one of those subtypes jumps to humans. The most watched strain right now is H5N1 bird flu. Sporadic human infections with H5N1 have occurred worldwide since 1997, historically with high mortality, usually following direct contact with infected poultry.
In the United States, a small number of human H5N1 cases have been reported since 2022, mostly among workers exposed to infected poultry or dairy cows without proper protective equipment. Most of these U.S. cases have been clinically mild, though a few hospitalizations and one death have occurred. Critically, no sustained person-to-person spread of H5N1 has been documented. The risk to the general public remains low, but health agencies closely monitor for genetic changes in the virus that could increase its ability to spread between people.
“Stomach Flu” Is Not Influenza
One common source of confusion: the illness people call “stomach flu” has nothing to do with influenza. Influenza is a respiratory infection that targets the nose, throat, and lungs. Viral gastroenteritis, the actual name for stomach flu, attacks the intestines and causes watery diarrhea, nausea, vomiting, and stomach cramps. It’s caused by entirely different viruses, most commonly norovirus or rotavirus. The shared nickname is misleading, but the two illnesses are biologically unrelated.