Influenza: Symptoms, Causes, & Risks From Flu Viruses

Katelyn Hagerty FNP

Medically reviewed by Kristin Hall, FNP

Written by Our Editorial Team

Last updated 11/27/2020

Influenza, more commonly referred to as the flu, is a contagious disease that affects your nose, throat and lungs. 

The flu is caused by influenza viruses, which can spread between people through tiny droplets released when people talk, sneeze or cough. 

There are different types of the flu, such as type A or type B, but all forms of the flu are caused by viruses. 

The flu can range in severity from mild to severe and, in some cases, lead to severe health complications and even death.

According to the CDC, about eight percent of all Americans will get sick with the flu every year, making it one of the most common viral infections. People of all ages may get the flu, although younger people have a higher risk of catching the flu than the elderly.

Below, we’ve listed the symptoms you may experience if you have the flu, as well as how it can differ from other common respiratory viral illnesses, such as the common cold. 

We’ve explained how influenza spreads and develops, as well as how it’s typically diagnosed by healthcare professionals. We’ve also listed treatment options that are available if you believe you have the flu, as well as ways to reduce your risk of catching the flu or passing it on to others.

Symptoms of Influenza (Flu)

Influenza can cause a variety of symptoms. In some people, these symptoms may be mild and short-lived. However, some pay may develop severe, persistent illness. Common symptoms of the flu include:

  • Fever or feverish/chills

  • Cough

  • Headaches

  • Sore throat

  • Stuffy or runny nose

  • Muscle and/or body aches

  • Extreme Fatigue

If you have the flu, you may not develop all of these symptoms. Some people with the flu notice a variety of symptoms, while others only may experience a few of the common symptoms listed above.

Many people with the flu develop a fever and/or chills, although not all cases of the flu involve this symptom. Some people with the flu may experience vomiting and diarrhea. This symptom tends to be more common in children than in adults and the elderly.

The symptoms of the flu tend to develop abruptly. If you have the flu, you may wake up in the morning and feel sick, even if you felt fine the day before. 

What Causes Influenza (Flu)?

The flu is caused by influenza viruses — contagious viruses that spread from person to person in a variety of ways. There are four types of influenza viruses: A, B, C and D. The seasonal flu that occurs every winter is typically caused by the human influenza A and B viruses.

Like other viral infections, the flu spreads from one infected person to others. A person with the flu can spread it to others through respiratory droplets that are released when the person talks, coughs or sneezes.

These droplets may be inhaled into the lungs by other people. They may also land on a nearby person’s mouth or nose. Experts believe that a person with the flu can spread it to other people within six feet through these droplets.

It’s also possible for the flu to spread through infected surfaces. If a person touches a surface, such as a counter or table, that has flu virus particles, then touches their face, it’s possible for the virus to make its way into their body.

You can catch the flu at any time of year. However, it’s most common during fall and winter. Flu infections in the United States usually peak from December to February, although flu season can last into May — a period in which other respiratory viruses, such as the common cold, are also common.

People with the flu may be able to infect others a day before they start to experience symptoms and may remain contagious for up to a week after becoming sick. 

Experts believe that the risk of a person spreading the flu virus is highest within the first three to four days after they begin to experience symptoms. People with weakened immune systems, as well as young children, may remain infectious for longer than others. 

If you catch the flu, you’ll typically begin to notice symptoms one to four days after the virus has made its way into your body. In some cases, people may become infected with the flu virus but not experience any noticeable symptoms.

Who Is Most at Risk?

Anyone can develop the flu. However, some people — such as those who live or work in shared facilities or have chronic illnesses — may have a higher risk of catching the flu than others. You may have an elevated risk of catching the flu if you: 

  • Live or work in a facility with lots of other residents, such as a military facility, dorm, nursing home or other shared environment with lots of people in close proximity.

  • Have a weakened immune system from use of certain medications, HIV/AIDS or organ transplant. This may also increase your risk of developing complications from the flu.

Virtual Primary Care

Connect with qualified healthcare providers online

Potential Complications Associated With Influenza (Flu)

Most cases of the flu get better over the course of one to two weeks. If you are young and in good health, it’s unlikely that a case of seasonal influenza will lead to any serious complications.

However, some people may develop serious complications from the flu that can prolong illness and, in some cases, even be deadly. Potential complications associated with the flu include:

  • Bacterial Pneumonia

  • Bronchitis

  • Sinus and ear infections

The flu can also worsen certain medical conditions. People with asthma may experience asthma attacks and worsened asthma symptoms while sick. People with heart disease who have the flu may also experience worsened heart disease symptoms.

Who Is Most at Risk of Complications?

Most people who get the flu will not experience complications and will recover at home, usually without any need for medical care or flu-specific medications. However, some people have an elevated risk of developing complications, including serious complications, from the flu. 

People with an elevated risk of experiencing complications from the flu include:

  • Pregnant women

  • Children younger than five years of age, and especially those under two years of age

  • People aged 65 years or older

  • People with diabetes

  • People with HIV/AIDs

  • People with cancer

  • People with asthma

  • People with heart disease and those who have had a stroke

The CDC provides detailed information for all of the groups listed above, including guides to the best ways to prevent flu and seek help if you need to speak to a medical professional. 

If you are in any of the categories listed above, it’s important to pay attention to your symptoms if you develop the flu. If you feel that your condition is worsening or that you’re not recovering at a normal speed, you should contact a medical professional for assistance. 

Emergency Warning Signs of Flu Complications

Most of the time, the flu won’t lead to serious complications. However, if you experience any of the following warning signs, you could be experiencing a serious complication from the flu and should seek medical help as soon as you can.

In children:

  • Bluish lips or face

  • Chest pain

  • Dehydration

  • Difficulty breathing and/or fast breathing

  • Fever and/or cough that improves, but later returns or worsens

  • Fever of 104°F or higher, or, in children 12 weeks or younger, any fever

  • Lack of alertness and/or interaction when awake

  • Ribs pulling in with each breath (retractions)

  • Seizures

  • Severe muscle pain

  • Worsening of chronic medical conditions

  • Any other serious or concerning medical symptoms

In adults:

  • Difficulty breathing and/or shortness of breath

  • Fever and/or cough that improves, but later returns of worsens

  • Lack of urination

  • Persistent confusion, dizziness and/or inability to arouse

  • Persistent pain and/or pressure in the chest and/or abdomen

  • Seizures

  • Severe muscle pain

  • Severe weakness and/or unsteadiness

  • Worsening of chronic medical conditions

  • Any other serious or concerning medical symptoms

Influenza (Flu) vs. The Common Cold

Influenza and the common cold are both respiratory illnesses caused by viruses. They share numerous common symptoms. Despite these similarities, there are several major differences between the flu and the common cold that can make it easy to tell one apart from the other:

  • Symptom onset. Flu symptoms tend to develop much more abruptly than the symptoms of the common cold, meaning you usually won’t notice a gradual sickness like you would if you have a common cold.

  • Severity. Although it’s definitely possible to catch a bald cold, cold symptoms are usually much less severe than the symptoms of the flu. Common colds are also far less likely to cause serious complications and health issues than the flu.

  • Fever. It’s generally rare to get a fever if you have a common cold. However, a fever of 100.4°F (38°C) is a common symptom of the flu. People with the flu may also get chills and sweats.

  • Other different symptoms. Some symptoms, such as a runny or stuffy nose, are more common in people affected by the common cold, but less likely in people with the flu.

Diagnosis of Influenza (Flu)

Your healthcare provider will usually diagnose influenza by asking you about your symptoms and conducting a physical exam. If you have flu symptoms, your healthcare provider may recommend taking a flu test to see if your symptoms are caused by influenza or a different type of illness.

The most common type of flu test is the rapid influenza diagnostic test, or RIDT. This type of test can identify influenza by detecting influenza antigens — a type of substance created by the virus that triggers an immune response in your body.

The rapid influenza diagnostic test usually provides results in 10 to 15 minutes. However, it’s not as accurate as other flu tests, meaning you may test negative even if you have the flu. 

If your healthcare provider still thinks that you have the flu after a negative RIDT, they may recommend a test called a rapid molecular assay. This type of test takes 15 to 20 minutes to provide a result and is more accurate at detecting the flu than the rapid influenza diagnostic test. 

In some cases, your healthcare provider may recommend a swab test. This method of testing takes longer to produce results — typically several hours — but is more accurate and sensitive than the faster flu testing methods.

Finally, if you have obvious, visible flu symptoms, your healthcare provider may be able to diagnose you with the flu without carrying out any testing. In fact, most people with flu symptoms aren’t tested, as the results of testing usually don’t affect how you’re treated.

Influenza (Flu) Treatment and Prevention

Getting sick with the flu isn’t a particularly pleasant experience. However, it’s a treatable illness that, for most people, passes on its own without the need for serious medical care or treatment with antiviral drugs.

If you notice flu symptoms and think you might have influenza, the best thing to do is to stay home. Do the following to make sure you don’t put others at risk of becoming sick:

  • If you have a fever, stay home for at least 24 hours after your fever is gone without the use of fever-reducing medicine. Avoid contact with others during this time.

  • Avoid going out to shop, travel or take part in social events, and only leave home if you need to seek medical care. If you need to leave your home, make sure to cover coughs and sneezes with a tissue. It may be beneficial to wear a facemask as well.

  • If you need to, call in sick to your workplace, school or other educational institution to avoid potentially spreading the flu to others.

Over-the-Counter Medications for Influenza (Flu) Symptoms

Most people will get better from the flu over the course of several days without the use of any medications. 

If you have the flu, some over-the-counter medications may help to ease the symptoms while your body fights the infection. According to the Cleveland Clinic, the following medications can help to relieve flu symptoms:

  • For fever and pain, analgesics such as acetaminophen (Tylenol®). Other analgesics that may be used to relieve flu-related fever and pain include ibuprofen (Advil®) and naproxen (Naprosyn®).

    Aspirin shouldn’t be used to treat flu symptoms, particularly in young people, as it may increase the risk of developing a potentially dangerous health condition called Reye's syndrome.

  • For a stuffy nose, decongestants such as pseudoephedrine (Sudafed®). People with hypertension (high blood pressure) should avoid these medications. Some people may experience insomnia, irritability and nervousness from decongestants.

  • For a persistent cough, antitussive medications. Antitussives include guaifenesin and dextromethorphan, which can be found in Robitussin® DM and a variety of other cough syrups, lozenges and tablets.

  • For a runny nose or sinus pressure, prescription nasal steroids such as fluticasone (Flonase®) or mometasone (Nasonex®), or over-the-counter antihistamines such as diphenhydramine (Benadryl®).

  • For dealing with a sore throat, over-the-counter cough lozenges such as Cepacol® and other brands, as well as the use of a homemade saltwater gargle several times a day.

  • For thinning and loosening cough/mucus production, guaifenesin (Robitussin®, Muco-Fen®, Humibid LA®, Mucinex®, Humibid E®).

  • For other flu symptoms, over-the-counter cold medications, such as Tylenol Cold & Sinus® or Nyquil®.

Although these medications won’t get rid of the flu virus, they can help to ease some symptoms and make your life easier while you recover at home. 

Some of these medications may cause side effects. It’s always best to consult with your healthcare provider before using any medication to treat flu or other illnesses, including over-the-counter products available at your local drug store or supermarket. 

Prescription Antiviral Medications for Influenza (Flu)

If you belong to a high-risk group for flu complications, contact your healthcare provider as soon as possible if you experience flu symptoms. To reduce your risk of experiencing complications, your healthcare provider may recommend the use of antiviral medications. 

Antiviral medications for flu are only available with a prescription. Currently, the FDA allows the use of four drugs to treat flu:

These medications help to reduce your risk of developing complications from the flu, including serious complications such as pneumonia. They work best when you start using them as soon as possible after you begin to experience flu symptoms, but no later than 48 hours after initial symptoms start.

If you’re at a high risk of developing complications from the flu, using antiviral medications can help to keep your symptoms mild, shorten the amount of time that you’re sick and lower your risk of requiring a hospital stay. 

Your healthcare provider will prescribe the most appropriate antiviral medication for you based on your flu symptoms and general health. 

Home Remedies and Lifestyle Changes

As we mentioned above, it’s important to stay at home when you have the flu. This is not only important for reducing your risk of infecting others, but also to give yourself a comfortable and quiet environment in which you can recover. 

While you’re at home, try the following approaches to ease your symptoms and recover:

  • Get plenty of rest. Getting over the flu requires a serious, prolonged effort from your immune system. Make sure to keep your activity level low and get lots of rest to help your body recover from illness.

  • Stay hydrated. Make sure that you drink plenty of liquids while you’re sick. Water, fruit juices and hot beverages can all help to keep you properly hydrated and make it easier to recover.


The easiest and most effective way to reduce your risk of catching the flu is to get a flu vaccine every year.

Getting the flu vaccine doesn’t completely guarantee that you won’t become sick. However, the most recent studies suggest that vaccination can reduce your risk of getting the flu by 40 to 60 percent during each flu season.

The flu vaccine is particularly important if you’re in a high-risk group for complications from the flu due to your age or a pre-existing health condition. 

Getting the flu vaccine is a fast, safe and convenient process. There are several different types of vaccine available, including a flu shot and nasal spray-based vaccine. All flu vaccines provide protection against the influenza virus within about two weeks. 

You can find the closest location for the flu vaccine by entering your zip code into the flu vaccine finder on the CDC website

In addition to getting a flu vaccine, it’s important to practice good health habits to lower your risk of being exposed to the influenza virus. This list from the National Institutes of Health is a useful resource for protecting yourself and others from viruses and other germs. It recommends:

  • Washing your hands often, especially before eating or preparing food, after using the toilet, after blowing your nose, coughing or sneezing, or after any situations that put you at risk of being exposed to viruses, bacteria and other germs.

  • Using an alcohol-based hand sanitizer to wash your hands when soap and water are not available.

  • Covering your mouth when coughing or sneezing, either with a tissue or the inside of your elbow.

Talk to a healthcare provider About Influenza (Flu)

Worried you might have the flu? Talk to a licensed healthcare provider now to learn more about what you can do to deal with the flu, from managing symptoms to antiviral medications that can speed up recovery and reduce your risk of developing complications. 

If appropriate, the provider can write you a prescription on the spot and send it directly to a local pharmacy of your choice, allowing you to get the relief you need fast, all without having to go to a healthcare provider’s office. 

This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

📫 Get updates from hims

Insider tips, early access and more.