Yes, the flu is caused by a virus. Specifically, influenza viruses belong to a family called Orthomyxoviridae, and they carry their genetic instructions in RNA rather than DNA. This distinction matters because it means antibiotics, which target bacteria, have no effect on the flu. It also explains why the flu changes from year to year and why you can catch it more than once.
Four Types of Influenza Virus
There are four types of influenza virus, labeled A through D, but only two of them cause the seasonal illness most people think of as “the flu.”
Influenza A and B are responsible for the winter flu seasons that sweep through populations every year. Of the two, influenza A is the more unpredictable. It’s the only type capable of causing pandemics, those worldwide outbreaks that happen when a dramatically new version of the virus emerges. Influenza A viruses are classified into subtypes based on two surface proteins, and there are 18 versions of one protein and 11 of the other, creating a large number of possible combinations. H1N1 and H3N2 are the subtypes currently circulating in people.
Influenza B doesn’t have subtypes but splits into two lineages: B/Victoria and B/Yamagata. It tends to cause slightly milder seasons than influenza A, though it can still lead to serious illness. Influenza C causes mild respiratory symptoms and doesn’t drive epidemics. Influenza D primarily infects cattle and isn’t known to make people sick.
How the Virus Gets Into Your Cells
Influenza viruses are covered in two key surface proteins that handle different stages of infection. The first acts like a grappling hook: it latches onto sugar molecules on the surface of cells lining your respiratory tract, which triggers the cell to pull the virus inside. Once the virus has hijacked the cell’s machinery to make copies of itself, the second surface protein takes over. It snips the connection between newly made virus particles and the host cell, freeing them to spread to neighboring cells and repeat the cycle.
The flu virus’s genome is split into eight separate segments, each carrying instructions for proteins the virus needs to enter cells, copy itself, or build its outer shell. This segmented design plays a critical role in how the virus evolves, because segments from different strains can shuffle together if two viruses infect the same cell at the same time.
Why the Flu Keeps Coming Back
The flu virus changes through two distinct processes, and understanding the difference explains why you need a new vaccine every year and why pandemics happen.
The first process is a slow, constant accumulation of small mutations. Every time the virus copies its RNA, minor errors creep in. Over months and years, these tiny changes alter the surface proteins enough that antibodies you built from a previous infection or vaccination no longer recognize the virus as well. This is why a flu strain from two winters ago may not match the one circulating today, even though it descended from the same lineage.
The second process is sudden and dramatic. It happens when a flu A virus from an animal population, such as birds or pigs, acquires the ability to infect humans. Because the surface proteins on this new virus look completely different from anything the human immune system has encountered, most people have little or no protection. This kind of abrupt change is what triggers pandemics. The 2009 H1N1 pandemic is a recent example.
How It Spreads and How Long You’re Contagious
After the virus enters your respiratory tract, symptoms typically appear about two days later, though the window ranges from one to four days. You can start spreading the virus to others a full day before you feel sick, which is one reason flu spreads so efficiently. You remain contagious for five to seven days after symptoms begin, with the first three days being the most infectious period.
Why Antibiotics Don’t Work on the Flu
Because the flu is a virus and not a bacterial infection, antibiotics are useless against it. Antibiotics work by targeting structures or processes unique to bacteria, such as cell walls or bacterial protein production. Viruses don’t have those structures. They hijack your own cells to reproduce, so there’s nothing for an antibiotic to attack.
Antiviral medications designed specifically for influenza work differently. They interfere with the virus’s ability to multiply inside your cells, particularly by blocking the surface protein that frees new virus particles from infected cells. These medications are most effective when taken within the first day or two of symptoms, because the virus replicates rapidly in that early window.
Complications From Flu Infection
For most healthy adults, the flu resolves on its own within a week or two. The more serious risk comes from what the virus does to your defenses while it’s active. Influenza damages the lining of your airways and temporarily suppresses certain immune functions, creating an opening for bacteria that are normally kept in check.
More than 20% of flu patients develop bacterial pneumonia as a secondary infection. This bacterial co-infection is the leading driver of severe illness and death among hospitalized flu patients. The most common culprits are bacteria that already live in or around your respiratory tract but can take hold once the virus has weakened the airway barrier and impaired immune cells’ ability to clear them. Young children, older adults, pregnant women, and people with chronic health conditions face the highest risk of these complications.
How Vaccines Keep Up With a Changing Virus
Because influenza mutates constantly, the vaccine is reformulated every year to match the strains researchers expect to circulate. For the 2025-2026 season, all flu vaccines are trivalent, meaning they protect against three viruses: two influenza A subtypes (H1N1 and H3N2) and one influenza B lineage (B/Victoria). The vaccine is available for anyone six months of age and older.
The specific virus strains chosen for the vaccine differ slightly depending on how the vaccine is manufactured. Some are grown in eggs, while others use cell-based or recombinant technology, and each method may use a slightly different reference strain. The goal in all cases is to train your immune system to recognize the surface proteins most likely to appear on circulating viruses that winter, buying you a meaningful degree of protection even if the match isn’t perfect.