The flu is caused by influenza viruses that infect the cells lining your respiratory tract. Four types of influenza virus exist (A, B, C, and D), but only types A and B drive the seasonal epidemics that sicken millions of people each winter. Understanding how these viruses work, spread, and change from year to year helps explain why the flu keeps coming back and why it hits harder in some seasons than others.
The Viruses Behind the Flu
Influenza A is the most consequential type. It infects humans, birds, pigs, and other animals, and it’s the only type capable of causing pandemics. Influenza B circulates only among humans and contributes to seasonal outbreaks but doesn’t trigger pandemics. Together, these two types are responsible for virtually all flu-related illness, hospitalization, and death each year.
Influenza C causes mild illness and doesn’t produce epidemics. Influenza D primarily infects cattle and has never been shown to cause illness in people.
How the Virus Infects Your Cells
The surface of every flu virus is studded with two key proteins. The first acts like a grappling hook: it latches onto sugar molecules on the surface of cells in your upper respiratory tract, pulling the virus inside. Once the virus has been absorbed into the cell, it hijacks the cell’s machinery to make copies of itself. The viral genetic material travels into the cell’s nucleus, where it tricks the cell into producing new viral components.
Those components are assembled into fresh virus particles, which migrate to the cell’s outer edge. The second surface protein then acts like a pair of scissors, snipping the new virus particles free so they can spread to neighboring cells. This cycle repeats rapidly, and within a day or two, enough cells are infected to trigger the immune response you experience as flu symptoms.
Human-adapted flu strains target cells in the upper respiratory tract, which is why the virus transmits so efficiently through coughing, sneezing, and breathing. Avian strains, by contrast, tend to latch onto cells deeper in the lungs. That’s why bird flu occasionally infects people who handle infected poultry but rarely spreads from person to person.
How the Flu Spreads Between People
Flu travels from person to person through three main routes. The most familiar is respiratory droplets: relatively large particles expelled when someone coughs, sneezes, or talks. These droplets typically settle to the ground within one to two meters. Smaller particles called aerosols, less than 5 micrometers across, can linger in the air for longer periods and retain the ability to infect. Both large and small particles carry infectious virus.
Surface contact plays a smaller role. Flu viruses can survive for hours on hard, non-porous surfaces like doorknobs and countertops, though they don’t last long on hands. Touching a contaminated surface and then touching your nose, mouth, or eyes can introduce the virus, but airborne routes are considered the primary driver of spread.
The Contagious Window
Symptoms typically begin about two days after infection, though the range is one to four days. What makes flu particularly hard to contain is that you can start spreading the virus a full day before you feel sick. You remain contagious for five to seven days after symptoms appear, with the first three days of illness being the most infectious period. Young children and people with weakened immune systems may shed the virus for even longer.
Why Flu Peaks in Winter
Flu season in temperate climates follows a strikingly consistent pattern, peaking during the coldest, driest months. Research published in the Proceedings of the National Academy of Sciences found that a single factor, absolute humidity (the total amount of moisture in the air), explains about 90% of the variation in how well the flu virus survives in airborne droplets.
When absolute humidity drops in winter, both indoors and outdoors, airborne flu particles dry out into tiny, lightweight nuclei that float longer and remain infectious. Warmer, more humid air does the opposite: it degrades the virus’s outer lipid coating, reducing its ability to infect. This is why flu circulates year-round in tropical regions with less dramatic humidity swings but explodes in sharp winter waves across places like North America and Europe.
Cold weather also pushes people indoors into closer contact, giving the virus shorter distances to travel between hosts. The combination of drier air and crowded indoor spaces creates ideal conditions for transmission.
Why You Can Get the Flu More Than Once
Flu viruses change constantly through a process called antigenic drift. Each time the virus copies itself, small mutations accumulate in its surface proteins. Over months and years, these mutations can alter the virus enough that antibodies from a previous infection or vaccination no longer recognize it well. This is the main reason flu vaccines are reformulated every year: the circulating strains shift just enough that last season’s protection fades.
A more dramatic change, called antigenic shift, happens when entirely different flu strains swap genetic material. This typically occurs when a virus from birds or pigs mixes with a human strain, often inside a pig that serves as a “mixing vessel” for multiple viruses. The result can be a radically new virus that almost no one has immunity to. Antigenic shift is rare, but it’s responsible for flu pandemics, including the 2009 H1N1 outbreak that originated in swine.
Animal Reservoirs and New Strains
Wild birds are the natural reservoir for influenza A. They carry a wide variety of viral subtypes, often without getting sick, and spread them through droppings and contaminated water. Domestic poultry can pick up these viruses and amplify them in densely packed flocks, occasionally producing highly dangerous strains.
Pigs are particularly important in flu ecology because their respiratory cells have receptors for both bird and human flu viruses. When a pig is infected with two different strains simultaneously, the viruses can exchange genetic segments and produce a hybrid strain capable of infecting humans. Most human infections from animal flu viruses come from direct contact with infected animals or contaminated environments like live bird markets and farms, especially during activities like handling, slaughtering, or culling. These spillover events are rare but closely monitored because any new strain with the ability to spread efficiently between people could spark a pandemic.
The fundamental cause of the flu is straightforward: a fast-replicating virus that exploits your respiratory tract. What makes it a persistent global threat is its ability to evolve, jump between species, and thrive in the dry winter air that brings people together indoors.