The flu is caused by influenza viruses, a family of respiratory viruses that infect the nose, throat, and lungs. Four types exist (A, B, C, and D), but only influenza A and B cause the seasonal outbreaks that sweep through communities each winter. In the United States alone, these viruses cause between 9.4 million and 51 million illnesses per year, depending on the severity of the season.
The Four Types of Influenza Virus
Influenza A is the most significant type. It’s the only one capable of triggering pandemics, those rare global waves of infection that occur when a dramatically new version of the virus emerges. The subtypes circulating in people right now are H1N1 and H3N2, named after two proteins that stud the virus’s outer surface. Influenza A also infects birds, pigs, and other animals, which is part of what makes it so unpredictable.
Influenza B circulates alongside A during flu season and can cause equally miserable illness. It doesn’t have subtypes like A does but is instead split into two lineages, called Victoria and Yamagata. The Yamagata lineage hasn’t been detected in circulation since March 2020, which is why the current season’s flu vaccines target only three strains instead of four.
Influenza C causes mild respiratory illness and doesn’t spark epidemics. Influenza D primarily affects cattle and isn’t known to make people sick. When someone says they “have the flu,” it’s almost always A or B.
How the Virus Gets Into Your Cells
Influenza is a respiratory virus, meaning it targets the cells lining your airways. The virus latches onto sugar molecules called sialic acid on the surface of those cells, using a protein on its own surface that acts like a key fitting into a lock. This binding triggers the cell to pull the virus inside through a normal recycling process the cell uses to absorb material from its surroundings.
Once inside, the virus unpacks its genetic material and smuggles it into the cell’s nucleus. There, it hijacks the cell’s machinery to churn out copies of itself. Those copies assemble near the cell surface and eventually break free, using a second surface protein that snips the connection to the host cell. Each infected cell can release thousands of new viral particles, which go on to infect neighboring cells. This cascade is what transforms a few inhaled virus particles into a full-blown infection within a day or two.
How It Spreads From Person to Person
Flu spreads mainly through respiratory droplets produced when an infected person coughs, sneezes, or talks. Symptoms typically begin about two days after exposure, though the window ranges from one to four days. What makes containment difficult is that people start shedding virus about a day before they feel sick, meaning you can spread it before you even know you have it. The most contagious period is the first three days of illness, but viral shedding can continue for five to seven days.
Winter is prime flu season for a reason that goes beyond people spending more time indoors. Research using animal models found that the virus transmits most efficiently at low relative humidity (20 to 35 percent) and cold temperatures around 5°C (41°F). At high humidity (80 percent), transmission was completely blocked, and at 30°C (86°F) it didn’t occur at all. Indoor heating during winter months creates exactly the dry, cool conditions the virus thrives in.
Why You Can Get the Flu More Than Once
Unlike some viruses that barely change from year to year, influenza is constantly evolving. It does this through two mechanisms that work on very different timescales.
The first, called antigenic drift, involves small, ongoing mutations that accumulate every time the virus copies itself. These tiny changes gradually alter the virus’s surface proteins enough that your immune system, which learned to recognize last year’s version, struggles to detect the new one. Drift is the reason flu vaccines are reviewed and updated every year, and why a bout of flu in 2022 doesn’t protect you from the 2025 strain.
The second mechanism, called antigenic shift, is rarer and more dramatic. This happens when an influenza A virus undergoes a major genetic overhaul, often by swapping genetic segments with a flu virus from an animal species like birds or pigs. The result can be a virus so different from anything the human immune system has seen that almost nobody has preexisting immunity. Antigenic shift is what causes pandemics. There have been four in the past century, including the 2009 H1N1 pandemic.
Flu vs. the Common Cold
The flu and the common cold are both respiratory infections, but they’re caused by entirely different viruses that even enter your cells through different doors. Cold viruses, most commonly rhinoviruses, latch onto a different receptor on cell surfaces than the sialic acid influenza uses. This distinction matters because it means having a cold doesn’t protect you from the flu, and vice versa.
The practical difference is intensity. Colds tend to build gradually with a runny nose and mild congestion. Flu typically hits harder and faster, with high fever, body aches, fatigue, and a dry cough that can knock you out for a week or more. The CDC estimates flu leads to 120,000 to 710,000 hospitalizations and 6,300 to 52,000 deaths annually in the United States, numbers that dwarf what common colds produce. That wide range reflects how much the severity of each flu season varies, largely depending on which strains are circulating and how well the population’s immunity matches up.
Why It Matters Which Strain Is Circulating
Not all flu seasons are equal. An H3N2-dominant season tends to be more severe than an H1N1 season, particularly for older adults. The specific combination of surface proteins determines how easily the virus spreads, how sick it makes people, and how well the vaccine works against it.
For the 2024-2025 season, flu vaccines in the United States are trivalent, meaning they target three virus components: two influenza A subtypes and one influenza B lineage. The shift from four-component to three-component vaccines happened because the B/Yamagata lineage appears to have disappeared from global circulation during the COVID-19 pandemic, making a fourth target unnecessary.
Influenza’s ability to constantly reinvent itself is what keeps it a persistent public health challenge. The same biological features that make it a relatively simple virus, just eight gene segments wrapped in a fatty envelope, also make it one of the most adaptable pathogens humans face.