Flu Symptoms: What Are the Signs of Influenza?

Flu Symptoms: What Are the Signs of Influenza? Fever, body aches, headache, fatigue, cough, sore throat, runny nose, vomiting, diarrhea, chills - know the common symptoms.

The flu, or influenza, is a serious illness caused by viruses.1 It’s different from the “stomach flu” that causes stomach issues. Symptoms start quickly and can include fever, aches, fatigue, and more.2 Some may also vomit and have diarrhea, especially kids. The flu’s symptoms are worse than a common cold.

It can lead to dangerous problems, mainly in at-risk groups.1 Testing can show if it’s the flu or another illness like COVID-19.

Key Takeaways

  • Influenza is a respiratory illness caused by viruses, not the “stomach flu”.
  • Common flu symptoms include fever, body aches, headache, fatigue, cough, sore throat, and runny nose.
  • Flu symptoms are typically more severe than cold symptoms and can lead to serious complications.
  • Diagnostic testing can help distinguish the flu from other respiratory illnesses like COVID-19.
  • Certain groups, such as the elderly and those with underlying health conditions, are at higher risk for flu complications.

What are the symptoms of the flu?

The flu often shows as fever, body aches, and a headache. You might feel very tired and have a cough, sore throat, and a runny nose.1 Fever is a key sign and can last 3-4 days.3 You’ll also likely have muscle and joint pains.3 Plus, headaches, tiredness, and a dry cough are common.3 Although less common, you could get a sore throat, runny nose, or even vomit or get diarrhea. This last one is more common in kids.3


Body Aches




Sore Throat

Runny Nose

Is it Flu or COVID-19?

It can be hard to spot the difference between the flu and COVID-19 by symptoms. Both can give you a fever, aches, and make you tired. They also affect your cough, throat, and nose.4 You need a special test to know for sure if it’s the flu or COVID-19. This usually means getting a swab from your nose or throat.

Knowing which you have matters. How we treat and handle each illness is not the same.

Similarities Between Flu and COVID-19 Symptoms

4 Losing the ability to taste or smell is common with COVID-19. It happens to about 38% to 55% of people.4 A loss of smell is felt by around 40% of those with COVID-19. Shortness of breath might be longer and worse with COVID-19.4 Its symptoms can go on for weeks or even months.

Diagnostic Testing to Differentiate

4 COVID-19 isn’t as common in kids, making up about 1% to 2% of cases globally.4 Having both flu and COVID-19 at the same time is rare.4 Getting a flu shot and doing what you can to prevent COVID-19 is advised. This helps stop both from spreading.

COVID-19 symptoms

Overview of Influenza

Influenza is a respiratory infection caused by viruses. It affects the nose, throat, and lungs.1 This is not the same as the “stomach flu” which causes diarrhea and vomiting. The “stomach flu” is a different illness that affects the gastrointestinal system.5 Most people with the flu will get better in 1-2 weeks without special treatment. For some, especially those at high risk, the flu can cause serious health problems.

Understanding what makes the flu different from other illnesses is crucial. It helps people know when to seek medical help.

Respiratory Infection Caused by a Virus

The flu comes from viruses that affect your nose, throat, and lungs.1 These viruses are easily spread from one person to another. This happens when an infected person coughs, sneezes, or talks.1

Different from Stomach “Flu”

It’s crucial to know the “stomach flu” isn’t the same as influenza.5 The “stomach flu” brings diarrhea and vomiting. But it’s a sickness that’s about the stomach and intestines, not the respiratory system like the flu.

High-Risk Groups for Flu Complications

Some people are more at risk of severe flu effects. This includes young kids, older adults, and pregnant women. Living in a nursing home or having a weak immune system also puts you at risk. Certain ethnic groups are also more vulnerable. They might get very sick with the flu, needing quick medical care. They could also develop severe complications, making the flu more dangerous for them.

Young Children and Older Adults

Kids under 12 months old, and people 65 years and older, are at risk16. This is because infants have the worst outcomes within the first 2 years of life. They face the highest chances of hospitalization and death under 6 months old6.

Pregnant Women

When women are pregnant, they’re more likely to have worse flu symptoms16. This risk lasts through flu season and up to two weeks after giving birth.

Nursing Home Residents

Living in a nursing home puts you at greater risk of flu complications1. This includes an increased chance of needing hospital care6.

People with Weakened Immune Systems

If your immune system is weak, the flu can be very serious for you1. This is especially true if you have conditions like HIV/AIDS or take immune-suppressing drugs6.

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Certain Racial and Ethnic Groups

In the U.S., people from certain backgrounds are more likely to get seriously ill from the flu1. This high-risk group includes Black, Latino, and American Indian people6.

Symptom Progression

Flu symptoms show up quickly, but cold symptoms take their time to appear.2 The flu brings on sudden fever, body aches, headache, and crushing tiredness. On the other hand, colds start with a slow runny nose, sneezing, and a light cough. Though both can cause a cough and sore throat, the flu hits harder with higher fever and worse aches.2 Telling the difference in how symptoms start and how bad they get helps spot if it’s just a cold or actually the flu.

Onset of Symptoms

The flu’s symptoms come all at once – fever, body aches, headache, and feeling totally worn out.2 This is very different from cold symptoms that show up slowly. Knowing how flu symptoms act fast can help you realize it might be more than just a common cold.

Distinguishing Flu from a Cold

Though the flu and colds can seem alike, the flu is usually worse, with a higher fever and intense body aches.2 Colds rarely cause a fever or bad body pain. They’re more about a stuffy or runny nose and constant sneezing. Recognizing these differences is key. It can alert you to seek medical help, especially for those more at risk, because the flu can bring serious problems.

When to See a Doctor

Most people with the flu can get better at home. Yet, some signs mean you should see a doctor right away.3

Emergency Warning Signs in Adults

For grown-ups, watch out for serious trouble breathing, chest pain, being dizzy a lot, seizures, or health issues getting worse.3

Emergency Warning Signs in Children

Kids might show signs like lips turning gray or blue, very pale skin, or not enough fluids. Take them to the doctor if this happens.3

If someone is at greater risk from the flu, like young children or elders, and they start feeling like they have the flu, they must get medical help fast.7 Early antiviral drugs can make the illness less severe and shorter.7

Causes of Influenza

The flu comes from influenza viruses. They move through the air in droplets from someone sick. When that person coughs, sneezes, or talks, they can spread the virus.8 You can also get the flu from surfaces. If you touch something with the virus on it, then touch your face, you might get sick.1

Respiratory Droplet Transmission

When a person with the flu coughs, sneezes, or talks, they spread tiny droplets. These droplets can enter the mouths or noses of people close by. Or, if they’re in the air, you might breathe them in. This is how the flu travels.8

Constantly Changing Flu Viruses

The flu viruses keep changing. Every year, new strains appear.9 That’s why the flu shot is updated often. It helps protect against the most common flu viruses for that year.1 However, if the flu changes a lot, your past flu or shot may not protect you.1

Risk Factors for Influenza

Some things can make someone more likely to get the flu or have worse symptoms.1 These include being young or old,1 living or working closely with others,1 and having a weakened immune system from illnesses like cancer.1 Other risks are certain health problems like asthma and heart disease,1 being pregnant,1 and obesity, especially a BMI of 40 or higher.1 People with these risks can get sicker from the flu and might need special care.


For those aged 12 months or younger, flu complications are more likely,1 as they are for adults 65 and older.1 Some groups in the U.S., such as American Indians and Latinos, are more likely to need hospital treatment for the flu than others.1

Living or Working Conditions

If you live or work closely with others, for example in nursing homes or hospitals, you have a higher chance of getting and spreading the flu.1

Weakened Immune System

If your immune system is weak, whether from diseases like cancer or HIV, or from medicines like corticosteroids, you are at a bigger risk from the flu.18

Chronic Illnesses

If you have ongoing health problems like asthma or diabetes, you’re also at higher risk from the flu.18 This holds true for chronic lung diseases too,8 and for certain metabolic and kidney diseases.8


Being pregnant, especially during the flu season, increases your risk for flu complications.1


Obesity, with a BMI of 40 or more, also makes flu complications more likely.18 Having a high BMI is a risk factor for getting severely ill from the flu too.8

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Flu Complications

Most people get better from the flu in 1-2 weeks. However, it can cause serious problems, especially for those at high risk.1 These can include pneumonia, bronchitis, asthma getting worse, heart issues, and ear infections.10 Pneumonia is a very serious complication that can lead to death, mainly in older adults and those with existing health issues.10 Getting prompt medical help is key to managing these complications.


Pneumonia can happen after the flu, showing signs like chills, fever, and chest pains. There’s also sweat, cough with green or bloody mucus, and shortness of breath.10 Bacterial pneumonia, a type that can come along with the flu, has worse symptoms. With bacterial pneumonia, you might have very high fevers, more coughing, and mucus that is tinged green.10 However, antibiotics only work on bacterial pneumonia.


The flu could turn into bronchitis, which means your bronchial tubes are inflamed. This leads to a constant cough, wheezing, and finding it hard to breathe.

Asthma Flare-ups

The flu can make asthma worse, causing episodes with symptoms like shortness of breath and chest tightness. It can also make managing asthma harder.10

Heart Problems

Having the flu can inflame your heart muscle, something called myocarditis. This could result in serious heart issues like failure or an irregular heartbeat.10

Ear Infections

Ear infections are another risk, especially for kids. They come with pain, fever, and for some, hearing loss.

Prevention and Vaccination

The best way to avoid the flu is through an annual flu shot.11 It greatly lowers your flu risk and keeps off serious complications.12

Importance of Annual Flu Vaccination

This year, it’s extra vital to get your flu shot. Both the flu and COVID-19 are spreading.12 A flu shot guards against both.

Types of Flu Vaccines Available

Flu vaccines come in many forms, like the shot or a nasal spray.12

Eligibility for Nasal Spray Vaccine

The nasal spray works for folks 2-49 years old. But, it’s not for everyone, especially those at high risk.12

Controlling the Spread of Influenza

After getting the flu shot, there are more ways to stop flu from spreading.13 Not just for flu, but for many germs, washing hands often with soap and water is a must.13 When soap isn’t around, using an alcohol hand rub also works well.13 Another big step is not coughing or sneezing into the air, but into a tissue or your elbow. Then, make sure to wash your hands.13 Disinfect commonly touched places, like doorknobs. This can also help stop the flu from moving around.13 Try to stay away from crowds during flu season. This can decrease your chance of catching the flu.

Hand Hygiene

Washing your hands often with soap and water is key in stopping the flu.13 It’s especially important to clean them after you cough, sneeze, or touch things that might have germs.

Cough and Sneeze Etiquette

When you cough or sneeze, always cover your mouth with a tissue or your elbow. After that, wash your hands right away.13 Doing this keeps the flu from spreading too easily.

Cleaning Surfaces

13 The flu virus can live on surfaces for two days. So, cleaning and disinfecting things people touch a lot is very important.13 This stops the virus from moving around in our homes and other places.

Avoiding Crowds

13 Since the flu spreads easily where many people are, it’s best to avoid such places during flu season.13 Staying apart from others can help slow down the flu. This keeps you and those around you safer.

Flu Complications for High-Risk Groups

Groups at risk include young kids, the elderly, pregnant women, and those with health issues or weak immune systems.8 They could get very sick with the flu, facing dangers like pneumonia and breathing failure.14

Pneumonia and Respiratory Failure

The flu isn’t just a bad cold. It can hurt the heart, brain, and muscles, making things worse for people already sick.14

Having asthma, heart problems, or diabetes makes the flu even scarier.14 It might cause lung issues, making it hard to breathe.14 So, quick help and medicine are key if you’re in a high-risk group.8

Inflammation of Heart, Brain, and Muscles

The CDC says most flu deaths and many hospital trips are among those 65 and over.14 Little ones under 5 are also in danger due to their growing immune systems.14

Babies under 6 months can’t get the shot, putting them at high risk.14 Moms-to-be can get very sick, but a flu shot helps protect them and their baby.14

Exacerbation of Chronic Conditions

Flu hits people with health issues harder.14 It can lead to pneumonia, which is hard on the lungs.14

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It might also cause bronchitis, making breathing tough.14 Dehydration is a big worry for the very young and the elderly.14 It can make other conditions, like asthma or heart issues, much worse.14

Distinguishing Flu from the Common Cold

Colds and the flu look similar at first but have key differences.15 The flu hits you suddenly, while colds start slow.15 Flu often brings a high fever over 100°F for 3-4 days. Colds rarely get that high.16 You’ll feel better from the flu in 2-5 days, compared to a cold’s 7-10 days.

Fever and Body Aches

15 With the flu, expect a bad, dry cough. For colds, it’s a cough bringing up phlegm.15 You might feel your muscles ache a lot with the flu, but not so much with a cold.17 The flu makes you very tired, with a high fever, aches, and chills, starting suddenly.

Sneezing and Stuffed Nose

15 Sometimes, the flu can give you a stuffy nose, but it’s more common with colds.15 You might sneeze with the flu, but it’s expected with colds.15 A sore throat might point to the flu, as colds are more known for it.

Knowing these signs can help decide if you need to see a doctor.16 The flu can get very serious, especially for some people.16 Pneumonia is a big risk, and flu season is from fall to spring the year after.


The flu is a serious illness caused by viruses. It can lead to severe symptoms and even hospitalization, especially for some people.18 Recognizing its common signs like fever and cough helps to know if it’s the flu or just a cold.19 Quick medical help, a yearly flu shot, and good hygiene like washing hands can lower flu risks.19 It’s also key to understand how the flu is different from other sicknesses. This knowledge helps us stay healthy, especially in flu season.18

About 200,000 people in the U.S. end up in the hospital due to the flu each year.18 Some groups, like kids or those with certain health issues, are at a higher risk.18 When new flu strains appear, they can cause global outbreaks with many deaths.18

Though the flu and COVID-19 may seem alike, it’s vital to get checked to be sure of the illness.20 Medicine to treat the flu works best within the first 48 hours of getting sick.18 To prevent the flu, getting a shot, keeping hands clean, and not getting too close to sick people are smart steps.19


What are the most common symptoms of the flu?

Fever, body aches, and headache are the main symptoms. You might also feel tired, have a cough, sore throat, and runny nose.

How do the symptoms of the flu compare to COVID-19?

The flu and COVID-19 are similar. They share fever and body aches, but they need different tests to tell them apart.

What is the difference between the flu and the “stomach flu”?

The flu happens in the nose, throat, and lungs. The “stomach flu” is a different virus. It causes diarrhea and vomiting.

Who is at higher risk for serious flu complications?

Some groups are at higher risk. This includs young children, the elderly, pregnant women, and those with weak immune systems.

How do flu symptoms differ from a common cold?

Flu symptoms hit suddenly. They cause higher fevers and worse aches than a cold. Cold symptoms appear more slowly.

When should someone seek medical attention for the flu?

If you have trouble breathing or serious chest pain, see a doctor. This is especially important for those more likely to have complications.

How does the flu virus spread?

The flu spreads through the air or by touching something with the virus on it. Then if you touch your face, you can get sick.

What factors increase the risk of getting the flu or developing severe illness?

Age is a risk. So are close living or working conditions. A weakened immune system and chronic health issues increase your risk too.

What are some of the serious complications that can arise from the flu?

Complications can range from pneumonia to worsening asthma. They can also cause heart problems and ear infections. Pneumonia is very serious and can lead to death.

How can the flu be prevented?

Getting a flu shot is the best prevention. There are shots and a nasal spray. Regular handwashing and avoiding large crowds can also help.

Source Links

  1. https://www.mayoclinic.org/diseases-conditions/flu/symptoms-causes/syc-20351719
  2. https://www.cdc.gov/flu/professionals/acip/clinical.htm
  3. https://www.mayoclinic.org/diseases-conditions/flu/expert-answers/flu-symptoms/faq-20057983
  4. https://www.nyp.org/medicalgroups/queens/for-patients/healthcare-articles/covid-19-vs-flu
  5. https://www.ncbi.nlm.nih.gov/books/NBK279459/
  6. https://www.marshfieldclinic.org/specialties/primary-care/flu/flu-high-risk
  7. https://www.healthline.com/health/influenza/when-to-see-doctor-flu
  8. https://www.lung.org/lung-health-diseases/lung-disease-lookup/influenza/symptoms-causes-and-risk
  9. https://www.healthdirect.gov.au/influenza-a-flu
  10. https://www.webmd.com/cold-and-flu/flu-complications
  11. https://www.cdc.gov/flu/about/keyfacts.htm
  12. https://www.mayoclinic.org/diseases-conditions/flu/in-depth/flu-shots/art-20048000
  13. https://www.cloroxpro.com/resource-center/influenza/
  14. https://www.bannerhealth.com/staying-well/health-and-wellness/wellness/flu/high-risk-and-complications
  15. https://www.healthpartners.com/blog/cold-vs-flu-how-to-spot-the-symptoms/
  16. https://www.webmd.com/cold-and-flu/flu-cold-symptoms
  17. https://www.nm.org/healthbeat/healthy-tips/cold-vs-flu
  18. https://www.uptodate.com/contents/influenza-symptoms-and-treatment-beyond-the-basics/print
  19. https://lagunabeachuc.com/flu-symptoms-and-treatment-guide/
  20. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4915903/