Influenza (Flu)

Medically Reviewed By William C. Lloyd III, MD, FACS

What is influenza?

Influenza—or the flu—is a contagious respiratory viral infection that affects the nose, throat and lungs. It spreads through droplets from the nose and mouth of someone who has the virus. You can “catch” the flu if you inhale these droplets or touch a surface where droplets have landed and then touch your eyes, nose or mouth. The influenza virus is around all year. While there are several types of flu viruses, types A and B cause seasonal flu epidemics.

In the United States, flu infections usually begin increasing in October, peak around February, and can stay at epidemic levels through May. The flu is very common and anyone can get it. On average, seasonal flu infects up to 45 million Americans, hospitalizes up to 810,000, and kills up to 61,000 annually. To help protect yourself and others, get the annual flu vaccine each fall.

Influenza symptoms include fever, chills, body aches, headache, fatigue, cough, sore throat, and a runny or stuffy nose. These symptoms usually begin quite suddenly. This makes it different from the common cold, which typically has a gradual start. It’s important to note that not everyone will have all of these symptoms.

The disease ranges from mild to severe. It’s possible to have the flu without fever and it’s also possible to have life-threatening complications from the infection. People at risk of developing a severe and even potentially fatal disease course include adults 65 and older, children under 5 years, and anyone of any age with a weakened immune system or chronic medical condition.

For otherwise healthy people, influenza treatment consists mainly of rest and treating symptoms. There are also antiviral medicines that can shorten the duration of your symptoms. However, you need to see your doctor within the first 48 hours of your symptoms to use them. People at high risk of developing influenza complications should contact their doctors as soon as they develop flu symptoms. Seek immediate medical care for potentially serious symptoms, such as shortness of breath, difficulty breathing, or chest pain.

What are the symptoms of influenza?

Influenza symptoms tend to hit you suddenly, unlike a cold that often builds and worsens. Some people have mild symptoms with the flu, while others have severe symptoms that drag them down for days or even a couple of weeks. It’s also possible to have the flu virus and be asymptomatic—lacking symptoms.

Common symptoms of influenza

People with the flu may have some, all or none of the common symptoms, which include:

  • Cough
  • Fever and chills
  • Runny or stuffy nose
  • Sore throat

Serious symptoms that might indicate a life-threatening condition

In some cases, influenza can cause life-threatening complications, such as pneumonia. Seek immediate medical care (call 911) if you, or someone you are with, have any of these serious symptoms including:

  • Bluish tinge to the lips, nails or skin
  • Chest pain, which may worsen with deep breathing
  • Fever lasting more than five days or that doesn’t respond to fever-reducing medicine within two hours

People at high risk of developing flu complications should contact their doctor as soon as they develop any symptoms of the flu. Do not wait for serious symptoms to occur.

What causes influenza?

Influenza viruses cause the influenza respiratory disease. There are two main types—influenza A and influenza B—that cause seasonal flu epidemics in humans. They are constantly changing—or mutating—into new A or B strains.

Influenza A viruses are more common, change or mutate faster, and cause disease in animals as well as humans, compared to influenza B viruses. This group is responsible for flu pandemics—or global outbreaks of the flu. When they come from an animal, people commonly name them after the animal. For example, the swine flu pandemic of 2009 was due to a flu virus that originates in pigs. However, scientists have a more formal naming system using the subtype letters 'H' and 'N.' The 2009 pandemic virus was an H1N1 type A flu. This virus, along with other type A viruses, continues to circulate seasonally in humans.

Influenza B viruses are less common and are relatively stable compared to the A viruses. This group is not known to infect animals or cause global outbreaks. They circulate seasonally in humans and vary geographically.

What are the risk factors for influenza?

Anyone can get influenza, as it is highly contagious. Being around someone with the flu puts you at risk of catching it. The flu spreads through respiratory droplets the infected person releases when coughing, sneezing or speaking. However, people with the virus can be contagious for about a day before symptoms develop. So, you can get the flu from someone who doesn’t look sick yet. People remain contagious for about a week after they become sick.

Respiratory droplets can travel about six feet from the person with the flu. The closer your contact with a sick person, the higher your risk of contracting the virus. These droplets can also fall on objects and contaminate them. You can get the virus if you touch these contaminated surfaces and then touch your eyes, nose or mouth.

Reducing your risk of influenza

With any contagious respiratory virus, your first line of defense is good personal and respiratory hygiene. You can lower your risk of getting (and spreading) the flu with these steps:

  • Avoid touching your eyes, nose and mouth with unwashed hands.
  • Cover your nose and mouth with a tissue when you cough or sneeze. Throw the tissue away immediately and wash your hands. If you don’t have a tissue, use the corner of your elbow.
  • Keep six feet of space between you and anyone who appears to be sick. If you are sick, stay at least six feet away from others.
  • Stay home if you don’t feel well or have a fever.
  • Wash your hands regularly and thoroughly, 20 seconds with warm, soapy water. A common suggestion for measuring the time is singing “Happy Birthday” twice. An alcohol-based sanitizer is a good substitute if you can’t wash your hands.

Encouraging those around you to follow these rules will help everyone stay healthy. However, the most effective way to prevent the flu is to get an annual flu vaccine. The vaccine changes each year, so it’s necessary to get one each and every year.

The CDC (Centers for Disease Control and Prevention) recommends everyone 6 months of age and older get the annual vaccine. There are different types of flu vaccines and the vaccine isn’t safe for everyone, such as those with allergies to its ingredients. So, talk with your doctor to find out if the vaccine is right for you and which one to get.

Every year, researchers study the different strains of the virus to make predictions about which strains will predominate in the coming flu season. Scientists use that information to make the vaccine. The technique isn’t perfect, so it is possible that a strain not in the vaccine will circulate widely. Still, getting the vaccine prevents millions of flu cases and saves thousands of lives each year.

It is also possible to catch the flu before the vaccine takes effect. In general, you need two weeks from the time you get the shot until your body builds up immunity. Getting the vaccine in the early fall offers the best chance of avoiding this situation.

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How is influenza treated?

The goals of influenza treatment are to treat the symptoms and prevent complications. If you are otherwise healthy and have a mild flu, you can manage it at home with rest, fluids, and over-the-counter medicines. You will likely feel better within a week or two. You should stay home from work, school and other places for a week from the time your symptoms started to prevent spreading the flu virus to other people.

See your doctor if you have severe or concerning symptoms. You should also see your doctor right away if you are in a high-risk group for influenza complications.

Antiviral medicines can help shorten the duration of your symptoms and prevent complications. But you need to start them within 48 hours of noticing symptoms.

Current antivirals for the flu include:

  • Baloxavir (Xofluza)
  • Peramivir (Rapivab)
  • Zanamivir (Relenza)

What are the potential complications of influenza?

Influenza can be serious and even deadly for people in certain groups. Hundreds of thousands of Americans end up in the hospital due to the flu each year. And tens of thousands die from it.

The flu can lead to asthma flares, bronchitis, pneumonia, and heart problems. The most serious of these is pneumonia and respiratory distress, which is often fatal for older adults with chronic conditions. Groups at risk of serious complications from the flu include:

  • Adults 65 years and older
  • Children younger than 5 especially very young children under 2 years of age
  • People with weakened immune systems or chronic medical conditions, such as asthma, chronic obstructive pulmonary disease, diabetes, heart problems, or morbid obesity
  • Pregnant women and postpartum women during the first two weeks
  • Residents of nursing homes and other long-term care facilities

If you are in a high-risk group, talk with your doctor about the flu. Find out if and when you should be vaccinated and what other vaccines you need to stay healthy. Make sure you know when to call your doctor if you develop symptoms and what the red flags are for seeking immediate medical care.

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  1. 2019-2020 U.S. Flu Season: Preliminary Burden Estimates. Centers for Disease Control and Prevention. 
  2. About Flu. Centers for Disease Control and Prevention. 
  3. Disease Burden of Influenza. Centers for Disease Control and Prevention. 
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  10. Types of Influenza Viruses. Centers for Disease Control and Prevention. 
  11. What You Should Know About Flu Antiviral Drugs. Centers for Disease Control and Prevention. 
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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Mar 24
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