What Is the Flu?

The flu is a respiratory infection that predominantly affects your lungs, causing other symptoms that can change and vary between individuals. Different strains of the flu virus emerge each year, making preventive practices and preparation of utmost importance.

During the colder winter months, when more people are spending time in indoor social settings, these types of viruses are more frequently and quickly spread compared to other times of the year.

This article will help you identify flu symptoms, understand how it spreads, and learn how to protect yourself from the flu.

sick woman sitting on a couch with a blanket

Roos Koole / Getty Images

Flu Symptoms

Flu symptoms can vary from person to person. They can also change in severity depending on the individual and the strain of the virus. For example, some people will develop a fever with the flu, while others will not.

Some of the most common symptoms of the flu include:

  • Fever and chills
  • Sore throat
  • Cough
  • Headaches
  • Tiredness
  • Shortness of breath
  • Congestion and runny nose
  • Body aches

Digestive symptoms, like vomiting and diarrhea, are more likely to occur among children than adults with the flu.

Flu vs. Cold vs. COVID-19

The flu, colds, and COVID-19 are all caused by viruses that can result in respiratory symptoms. Depending on the severity of your symptoms, it can be challenging to determine which one you have without taking a test.

The table below outlines the main differences between the flu, cold, and COVID-19 symptoms.

Flu vs. Cold vs. COVID-19 Symptoms
Symptoms Flu Cold COVID-19
Headache Common Rare Common
Sore throat Common Common Common
Runny nose Common Common Common
Muscle aches Common Sometimes Common
Diarrhea Sometimes (mostly kids) No Sometimes
Cough Common Common Common (usually dry)
Nausea, vomiting Sometimes (mostly kids) No Sometimes
Loss of sense of smell or taste Rare Sometimes (with congestion) Common (without congestion)
Tiredness Common Sometimes Common
Fever Common Sometimes Common
Shortness of breath Common Rare Common
Sneezing Rare Common Rare

Flu A vs. Flu B

There are four flu types: A, B, C, and D. Influenza C usually only causes mild infections in people, and influenza D primarily causes infections among cattle. Only influenzas A and B are known to cause flu pandemics and regularly cause seasonal epidemics every winter in the United States.

Influenzas A and B are treated the same way and can have similar symptoms, but they have some differences in how they affect people.

Flu A is much more common, responsible for approximately 75% of all flu cases experienced yearly. This is likely because it can be transferred from animals and birds to humans and mutates (changes) quickly. As a result, creating vaccines to keep up with new strains every year is more challenging with flu A.

Influenza A is the only type of flu that causes pandemics. It can affect adults and children but tends to be more severe in adults. Finally, flu A is more prevalent at the beginning of cold and flu season, around January.

Flu B only affects humans and, more commonly, infects children. While it usually has mild symptoms, it can have more severe effects on kids under age 5. It tends to be more prevalent later in cold and flu season, around February and March.

As for symptoms, both flu A and flu B tend to manifest with the following:

What Causes the Flu?

The flu is caused by various influenza virus strains passed between humans and, in some cases, from animals to humans. The viruses can spread fast across the globe, especially flu A, because of their ability to mutate quickly. This is why flu A viruses can cause a worldwide pandemic.

For this reason, more flu cases are seen during the year's colder months, dubbed "cold and flu season." This is because more people spend time indoors and closer to others more often than during the warmer months.

How the Flu Is Spread

Different strains of these contagious viruses circulate every year. Flus A and B are easily transmitted (spread) when someone with the virus sneezes or coughs into the air or onto a surface that someone else inhales or transfers to their eyes, nose, or mouth. Someone with the flu virus is most likely to transfer it to someone else within the first few days of getting symptoms.

Flu A can be spread between humans or from animals and birds to humans, though the latter is rare. Flu B is only transferred between people. The virus is spread through respiratory particles in the air or on contaminated surfaces.

When Is Flu Season?

Flu viruses circulate all year long. However, flu season in the United States generally begins in the fall and lasts throughout the winter. Peak flu season tends to occur from around October through February. During these colder months, more flu cases are seen due to increased transmission rates between people.

Flu Complications and Risks

Most people will recover from the flu within a few days to two weeks. Unfortunately, some people can experience flu complications, which can become life-threatening.

Moderate complications of the flu can include ear and sinus infections. Serious complications can include pneumonia, an infection that causes inflammation of the air sacs in your lungs, which may fill with fluid or pus.

Additionally, inflammation of the heart, brain, or muscle tissues can occur. For some people, if left untreated, flu can lead to multi-organ failure or an extreme whole-body inflammatory response called sepsis.

People with chronic health problems can experience worsened symptoms of the existing condition. For instance, the flu can lead to more severe asthma attacks among people who already have this respiratory condition.

Higher-risk populations for flu complications include:

  • People 65 and older
  • Kids younger than five years old and babies
  • Pregnant people
  • Individuals with chronic medical conditions, such as diabetes, heart disease, or asthma
  • People with a weakened immune system

Flu Treatment

If you become sick with the flu virus, there are some measures you can take to help speed up your recovery as you ride the illness out.

Antiviral medications may help reduce the duration of symptoms and prevent flu complications if you start taking them within the first few days of symptoms. A healthcare provider can prescribe these.

Otherwise, the best thing for you to do if you have the flu is to stay home and rest. Avoid contact with others unless you require medical care or are in a higher-risk group for complications. Avoid being around others until you are fever-free for 24 hours and without other symptoms. If you need to be around others, wear a mask to help prevent the spread of germs.

Home Remedies for Flu Recovery

If you are home resting with the flu, stay hydrated with fluids like water, juice, herbal tea, popsicles, broth, or electrolyte drinks, especially if you are experiencing vomiting or diarrhea. Make a warm saltwater mix to gargle and soothe a sore throat. If you're congested, it may help to keep a humidifier going by your bed at night.

How Long Does the Flu Last?

For most healthy people, symptoms of the flu should subside within a few days to less than two weeks. For people who are in a higher-risk group or have other existing medical conditions, the flu may cause these to worsen or may lead to complications that require immediate medical care.

How Long Is the Flu Contagious?

It's important to practice extra caution during cold and flu season because it is so easily transmitted. If you pick up the flu virus, you can transmit it to someone else one day before your symptoms begin and up to seven days after you improve. The most contagious period is the first three to four days after you start feeling sick.

Prevent the Flu With a Flu Shot

One of the most effective flu prevention methods is getting an annual flu shot. While the most prominent circulating flu virus strains change yearly, scientists do their best to predict which ones will be the most contagious using research. Annual vaccines are designed around these data.

Regardless, all flu vaccines in the United States protect you against four different flu viruses. These include an influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses.

Flu shots work by causing your body to produce antibodies within two weeks of receiving the shot. These antibodies are then prepared to help you fight off flu virus germs you're exposed to during that flu season.

How Effective Are Flu Vaccines?

The Centers for Disease Control and Prevention (CDC) conducts annual studies to determine how effective the flu vaccine is each year. The most recent data suggests a 40% to 60% lower risk of getting the flu among people who get their annual flu shot. One of the most significant factors in its effectiveness is how well-matched the vaccine is with the most prevalent virus strains each year.

A healthcare provider can answer any questions about the flu shot and determine which one is best for you, especially if you are over 65. Older individuals are at a higher risk for complications from the flu. They should receive a higher dose or an adjuvanted (an ingredient to produce a better immune response) flu vaccine, including the Fluzone High-Dose Quadrivalent, Flublok Quadrivalent, or Fluad Quadrivalent vaccine.

The best time to get your flu shot is in September or October, so your body has time to prepare antibodies before peak flu season arrives. Annual flu shots are available at many pharmacies, grocery stores, and primary care offices.

15 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Lauren Panoff, MPH, RD
Lauren Panoff, MPH, RD, is a plant-based dietitian, writer, and speaker who specializes in helping people bring more plants to their plate. She's a highly respected writer in the health and nutrition space and loves talking about the power of diet. Lauren aims to connect people with the information and resources to live their healthiest, fullest life.