Key takeaways
Influenza, also known as the flu, is one of the most common respiratory diseases in the world. Each year, around a billion cases of the flu occur worldwide, so it’s very likely you’ve been exposed to the flu—or had symptoms of the flu—at least once in your life.
Because the flu is so common, it can be easy to underestimate the severity of flu symptoms. For most healthy adults, the flu may feel like a more intense cold, but the flu can cause severe illness or lead to complications, especially for people at higher risk.
What is the flu?
The flu is a highly contagious respiratory disease caused by influenza viruses, peaking during the colder months in what’s known as flu season. The flu spreads through respiratory droplets created when someone with the flu speaks, coughs, or sneezes. If you inhale these droplets, the flu virus can enter your respiratory tract (including organs like the mouth, nose, throat, and lungs) and begin replicating, which typically leads to illness.
What are the most common flu symptoms?
Flu symptoms typically begin within 1 to 4 days of exposure to the virus. Unlike the common cold, in which symptoms develop over the course of a few days, flu symptoms tend to hit suddenly.
As the days progress, you may experience symptoms like:
- Feeling feverish or having chills
- Cough
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Headaches
- Fatigue (mental, physical, or emotional exhaustion not relieved by rest)
- Tiredness
- Vomiting or diarrhea (more common in children than adults)
Not everyone with the flu will experience the same severity. Like any illness you may contract, your body will respond differently to the virus based on your overall health, age, immune history, and preventive care like vaccination.
If you are unsure whether your symptoms are the flu or a different illness, visit Am I sick? to read more about how flu symptoms compare with other common illnesses.
When to seek emergency care for the flu
The flu is usually mild, but illness can become severe and, in some cases, life-threatening, especially for people at higher risk. If you experience any of these warning signs, you should seek medical care immediately.
Seek emergency care for children if they experience:
- Trouble breathing
- Bluish lips or face
- Retraction (ribs pulling in while breathing)
- Chest pain
- Severe muscle pain
- Dehydration (no urination for 8 hours, dry mouth, or inability to produce tears when crying)
- Lack of awareness/alertness
- Seizures
- Fever above 104°F that is not helped by medication
- Any fever if a child is younger than 12 weeks
- Fever or cough that improves but returns or worsens
- Worsening of chronic medical conditions
Adults should seek emergency care if they experience:
- Trouble breathing or shortness of breath
- Persistent pain or pressure in the chest
- Persistent dizziness or confusion
- Seizures
- Not urinating/dehydration
- Severe muscle pain
- Severe weakness
- Fever or cough that improves but returns or worsens
- Worsening of chronic medical conditions
Who is at the highest risk for flu symptoms becoming severe?
In most healthy individuals, the flu is mild and recovery is expected within a few days to two weeks, but it's still important to take the flu seriously. The flu can lead to serious complications for anyone, but especially groups at higher risk, such as:
- Adults 65 and older
- Young children under the age of 5 (and especially young children under 2)
- Pregnant women
- People with certain chronic conditions, such as asthma, diabetes, or heart disease
- People with weakened immune systems
High-risk populations are more likely to experience severe illness because their bodies are less able to fight off the virus or withstand the added strain of infection. For high-risk individuals, vaccination can help prevent flu-related illness and potentially serious flu complications that may lead to hospitalization, or in some cases, death. The CDC recommends yearly vaccinations for all people 6 months and older with rare exceptions.
To check your flu vaccine eligibility and book a vaccination appointment, visit VaxAssist by Pfizer.
What are the serious complications of the flu?
Although the flu is typically a mild to moderate illness, it can still lead to severe complications, especially for those at higher risk. One common and serious complication of the flu is pneumonia, a potentially life-threatening infection causing inflammation in the lungs.
Other serious complications caused by the flu can include inflammation of the heart (myocarditis), brain (encephalitis), or muscle tissues (myositis, rhabdomyolysis), and organ failure.
Infection of the respiratory tract can even trigger a severe inflammatory response in the body that could lead to sepsis, the body's life-threatening response to infections.
Flu can also worsen chronic conditions. People with asthma may have attacks while sick, and those with chronic heart disease may see their condition worsen.
How is the flu treated?
For those with an active infection, prescription treatment options are available for the flu. Antivirals work by fighting the spread of flu viruses in your body, which can help shorten the time you are sick and help prevent some complications. However, most people experience mild illness and do not need medical care or antiviral drugs. Talk to a healthcare provider to see if treatment is right for you.
If symptoms are mild, flu treatment can often be focused on symptom relief. Prioritize rest and hydration when ill with the flu and take fever reducers (like acetaminophen) when necessary.
The best protection starts before flu season with vaccination. Vaccination can help lower the risk of getting sick from common strains of flu viruses and developing serious complications. For high-risk individuals especially, vaccination can help reduce the likelihood of flu-related worsening of chronic conditions and flu-associated hospitalization.
Types of flu viruses
If you've ever wondered why the flu seems different from year to year, or why there's a new flu shot every season, it starts with understanding that "the flu" isn't just one virus. There are four types of influenza viruses, classified as types A, B, C, and D, each with distinct differences from one another. Of these different viruses, only influenzas A & B have been known to cause seasonal epidemics.
Influenza A
Influenza A is the most common type of flu, typically making up the majority of seasonal flu cases. It's sorted into subtypes based on the proteins found on the surface of the virus. That's where labels like A(H1N1) and A(H3N2) come from: the "H" and "N" each stand for a type of surface protein—hemagglutinin (H) and neuraminidase (N)—and the numbers tell you which subtype of each protein the virus has.
These subtypes branch off into groups and sub-groups called clades and sub-clades, with over 130 subtype combinations identified in the wild, though only some circulate in people. Right now, the two main subtypes circulating in people are A(H1N1) and A(H3N2).
During the 2025-2026 flu season, a new variant of influenza A(H3N2) called subclade K spread quickly and became the most common flu virus in the United States, accounting for roughly 90% of A(H3N2) viruses.
Influenza A is the only type of influenza known to have caused pandemics, which are the worldwide spread of a new infectious disease across countries or continents.
Influenza B
Influenza B is less common than influenza A, but it circulates every flu season and can cause serious illness. Like influenza A’s subtypes, influenza B can be broken down further into groups called lineages. These lineages, called B/Victoria and B/Yamagata, are named after the locations where the strains were first isolated: Victoria, Australia, and Yamagata, Japan.
Historically, B/Victoria has more commonly affected younger individuals, whereas B/Yamagata was more common among older adults. But as of March 2020, B/Yamagata viruses have not been detected by global flu surveillance, and scientists believe the lineage could be extinct.
Because B/Yamagata is no longer circulating, it was removed from U.S. flu vaccines beginning with the 2024–2025 season. Today's flu vaccines now protect against three viruses: one influenza A(H1N1) virus, one A(H3N2) virus, and one B/Victoria virus.
Influenza C & D
Influenza C and Influenza D are much less prevalent variants of influenza that aren’t typically protected against by any vaccines. Influenza C is not believed to cause seasonal epidemics and generally causes mild illness with cold-like symptoms in humans (such as cough, runny nose, and hoarseness). Influenza D primarily affects cattle and is not known to cause illness in humans.
Preparing for the next flu season
Knowing how to recognize flu symptoms, when to seek emergency care, and what treatment options are available can help you protect yourself and feel more prepared for flu season.
For most people, the best way to help reduce the risk of getting sick or experiencing serious complications starts with yearly flu vaccination. If you have questions about the flu, your risk, or whether vaccination may be right for you, talk with a healthcare provider who can offer guidance based on your individual health.
Ready to take the next step? To check your flu vaccine eligibility and book a vaccination appointment, visit VaxAssist by Pfizer.
Frequently
asked questions
How does the flu spread from person to person?
The flu primarily spreads through small droplets in the air, or in some cases, on surfaces. These droplets are made when people with the flu cough, sneeze, or talk, and the virus itself spreads when people come into contact with these droplets.
How do doctors diagnose the flu?
Depending on your provider, the flu may be diagnosed using tests or by clinical judgment. Flu tests work by swabbing the inside of the nose or throat to test for the virus (similar to a COVID-19 test), while clinical judgment is a decision-making process by your doctor based on the symptoms you are experiencing.
Why do flu viruses change from one season to the next?
Like other viruses, flu can change over time through a process called “antigenic drift.” When a flu virus infects someone, it replicates by making copies of itself, and each time it copies its genes, small changes (mutations) can happen.
Flu viruses copy their genes using RNA instead of DNA. RNA copying doesn't catch and fix mistakes the way DNA copying does, so mutations build up quickly and change the virus over time. As the virus changes, the body's immune system may not recognize it as easily. This lets the virus infect people who were protected against it before. This is why you can catch the flu more than once, even if you've had it before. It's also why the flu vaccine is updated every year to keep up with the changing virus.
Why does flu season usually fall in autumn and winter?
Flu season in the United States typically begins in October and lasts through April due to factors like colder weather, indoor crowding, school sessions, and more.
Continue reading about why flu season happens.
Does having the flu once make you immune to getting it again?
Having the flu once does not make you immune to reinfection. When you become ill with the flu, your body builds immunity against that specific virus, but flu viruses change (mutate) over time, and your existing antibodies are less effective at recognizing and preventing illness from these mutated viruses.
There are also multiple types and strains of flu viruses that circulate, and immunity to one doesn’t protect you from others. Because these viruses change each year, the flu vaccine is updated annually to help protect against the strains expected to be most common.
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