Influenza A is the most common and most dangerous type of flu virus, responsible for seasonal epidemics and all known flu pandemics. It infects a wide range of hosts, including humans, birds, pigs, and other mammals, which gives it a unique ability to change dramatically and cause widespread illness. The 2024-2025 flu season in the United States was one of the most severe since 2017, with an estimated 82 million illnesses, 1.3 million hospitalizations, and 130,000 deaths.
How Influenza A Differs From Other Flu Types
There are four types of influenza virus: A, B, C, and D. Influenza A and B cause the seasonal flu epidemics you hear about each winter, but influenza A is the only type that triggers pandemics. That’s because it circulates in animals as well as humans, giving it a much larger genetic playground for developing new strains. Influenza C causes only mild illness, and influenza D primarily affects cattle.
Influenza A viruses are classified by two proteins on their surface: hemagglutinin (H) and neuraminidase (N). There are 18 known H subtypes and 11 known N subtypes, creating a vast number of possible combinations. More than 130 subtype combinations have been identified in nature, mostly in wild birds. The two subtypes that routinely circulate in people are A(H1N1) and A(H3N2).
Why It Changes Every Year
Influenza A has a segmented genome, meaning its genetic material is split across eight separate strands of RNA. This structure is central to why the virus is so unpredictable. It changes through two distinct processes.
The first, called antigenic drift, happens constantly. As the virus copies itself, small mutations accumulate in the genes that code for its surface proteins. Over time, these tiny changes add up until your immune system’s antibodies from a previous infection or vaccination no longer recognize the virus well. This is the main reason you can catch the flu more than once and why flu vaccines are updated every year.
The second process, antigenic shift, is rarer and far more consequential. This is an abrupt, major change that produces a flu virus with entirely new surface proteins. It can happen when an animal flu virus gains the ability to infect humans, or when genetic material from different flu strains mixes together. Because most people have little or no immunity to the resulting virus, antigenic shift can spark a pandemic. There have been four flu pandemics in the past 100 years, and all were caused by influenza A.
How It Spreads
Influenza A spreads primarily through respiratory droplets when an infected person coughs or sneezes. This typically requires being within about six feet of the sick person. In confined spaces with limited airflow, smaller airborne particles can also transmit the virus over greater distances. You can also pick it up indirectly by touching a surface contaminated with the virus and then touching your face.
After exposure, symptoms usually appear within one to four days, with two days being the most typical incubation period.
Symptoms and Duration
Influenza A symptoms come on suddenly and tend to hit harder than a common cold. Most people experience some combination of fever or chills, cough, sore throat, runny or stuffy nose, muscle and body aches, headaches, and fatigue. Not everyone develops a fever. Some people, particularly children, may also have vomiting and diarrhea.
It can be difficult to distinguish the flu from other respiratory infections based on symptoms alone. Rapid diagnostic tests, available at most clinics, can confirm whether you have influenza A specifically. Most healthy adults recover within one to two weeks, though fatigue can linger longer.
Who Faces the Greatest Risk
During recent flu seasons, 9 out of 10 people hospitalized with the flu had at least one underlying health condition. The groups at highest risk for serious complications include adults 65 and older, children younger than 2 (with infants under 6 months facing the highest hospitalization and death rates), pregnant women (including up to two weeks postpartum), and people with weakened immune systems.
Chronic conditions that raise your risk include asthma, COPD, heart disease, diabetes, kidney or liver disorders, sickle cell disease, and neurological conditions. Having a BMI of 40 or higher also increases the likelihood of severe illness. People from certain racial and ethnic groups, including non-Hispanic Black, Hispanic or Latino, and American Indian or Alaska Native populations, face higher hospitalization rates.
Serious complications can include pneumonia, inflammation of the heart or brain, organ failure, and worsening of existing chronic conditions.
Treatment Options
Antiviral medications are available and work best when started within 48 hours of symptom onset. The most commonly used option is an oral antiviral taken twice daily for five days. A newer alternative works with a single dose, which can be more convenient. Both are effective against influenza A and B.
These medications shorten the duration of illness and reduce the risk of complications, particularly for people in high-risk groups. They don’t replace vaccination, but they’re an important second line of defense. Antivirals can also be used preventively after a known exposure, typically for seven days following contact with an infected person.
Annual Vaccination
Because influenza A continuously mutates through antigenic drift, flu vaccine formulations are reviewed and updated each year. For the 2025-2026 U.S. flu season, vaccines contain updated versions of both the A(H1N1) and A(H3N2) strains. The specific virus strains selected differ slightly depending on how the vaccine is manufactured: egg-based vaccines use one pair of reference viruses, while cell-based and recombinant vaccines use another, though both target the same circulating subtypes.
Annual vaccination remains the most effective way to reduce your chances of getting the flu and, if you do get sick, to lower the severity of illness.
The Animal Connection
Influenza A’s ability to infect birds, pigs, and other animals is what makes it uniquely dangerous among flu viruses. Wild birds are the natural reservoir for most subtypes, and the virus can jump to humans through direct contact with infected animals or contaminated environments. Live bird markets, poultry processing, and working on animal farms are recognized risk factors.
Avian influenza A(H5N1) first infected humans during a poultry outbreak in Hong Kong in 1997 and has continued to cause sporadic human cases in the decades since. A(H7N9) emerged in China in 2013. Swine flu viruses of the A(H1) and A(H3) subtypes occasionally infect people who have close contact with pigs. These zoonotic infections are concerning not because they currently spread easily between people, but because any animal flu virus that adapts to efficient human-to-human transmission could trigger the next pandemic through antigenic shift.