Influenza

Influenza is a contagious respiratory illness caused by viruses that can make you feel very sick, and in some cases, lead to serious complications or even death. The good news is that getting vaccinated every year and taking simple preventive steps can protect you and those around you.

Table of contents

• What is influenza?
• Signs and symptoms
• What causes influenza?
• How influenza spreads
• Who is at higher risk?
• How influenza is diagnosed
• Treatment options
• Prevention and vaccination
• What to do if you get sick

What is influenza?

Influenza, commonly called the flu, is a contagious illness caused by influenza viruses (germs that infect the nose, throat, and sometimes the lungs). It is not the same as the common cold or “stomach flu.”^[1][2]

The flu can cause mild to severe illness. While many people recover on their own within a week, influenza can lead to serious health problems including pneumonia (a lung infection), dehydration (not having enough fluids in the body), and worsening of other medical conditions you may have, such as heart disease or diabetes. At times, the flu can even lead to death.^[1][4]

Worldwide, there are around a billion cases of seasonal influenza every year, including 3 to 5 million cases of severe illness. The flu causes between 290,000 and 650,000 deaths from breathing problems each year.^[4]

• Nose
• Throat
• Lungs
• Respiratory system

Signs and symptoms

Flu symptoms usually come on suddenly, within one to four days after you come in contact with the virus. You often feel much worse with the flu than with a cold.^[1][2]

People who have the flu may experience some or all of these symptoms:^[1][3][4]

• Fever or feeling feverish with chills (though not everyone with flu will have a fever)
• Cough, usually dry
• Sore throat
• Runny or stuffy nose
• Muscle or body aches
• Headaches
• Extreme tiredness and weakness (fatigue)
• Vomiting and diarrhea (more common in children than adults)

Some people with influenza virus infections do not develop any symptoms at all. Research found that about 8 percent of people who tested positive for flu did not have symptoms.^[1]

Flu symptoms often begin suddenly and are usually more severe than cold symptoms. With a cold, you are more likely to have a runny or stuffy nose, and you can usually keep up with your normal activities. With the flu, symptoms can make you feel awful and stop you from doing your daily activities.^[2][3]

What causes influenza?

The flu is caused by influenza viruses. There are four types of influenza viruses: types A, B, C, and D.^[4][6]

Influenza A and Influenza B viruses spread among people and cause seasonal outbreaks of disease every year, particularly during winter months. These are the viruses that cause flu season. Currently, the main influenza A viruses spreading in humans are subtypes called A(H1N1) and A(H3N2). Influenza B viruses belong to either the B/Yamagata or B/Victoria lineage.^[4][3]

Influenza C virus usually causes only mild infections, mainly in children, and is not considered a major public health concern. Influenza D virus mainly affects cattle and is not known to infect or cause illness in people.^[4]

Only influenza type A viruses are known to have caused pandemics (worldwide outbreaks). H1N1 (sometimes called “swine flu”) and bird flu are both subtypes of influenza A.^[3][4]

How influenza spreads

The flu is very contagious, meaning it spreads easily from person to person.^[1][5]

Most experts believe that flu viruses spread mainly through tiny droplets that come out when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby or be breathed in. Less often, a person might get flu by touching a surface or object that has flu virus on it, then touching their own mouth, nose, or eyes.^[1][3]

You may be able to spread flu to someone else before you even know you are sick. People with flu are most contagious during the first three days of their illness. Some otherwise healthy adults may be able to infect others beginning one day before symptoms develop and up to five to seven days after becoming sick. Some people, especially young children and people with weakened immune systems, may be contagious for even longer periods of time.^[1]

The time from when a person is exposed to the influenza virus to when symptoms begin is usually about two days, but can range from about one to four days.^[1]

Who is at higher risk?

Anyone can get the flu, including healthy people, and serious problems from flu can happen at any age. However, some people are at higher risk of developing serious flu-related complications if they get sick.^[1]

You are at higher risk for serious illness from the flu if you:^[1][3][5]

• Are 65 years of age or older
• Are younger than 5 years old (especially children under 2 years)
• Are pregnant
• Have certain chronic medical conditions such as asthma, diabetes, heart disease, chronic lung disease (like COPD), kidney disease, liver disease, or neurological conditions
• Have a weakened immune system (from HIV/AIDS, cancer, or medications that suppress the immune system)
• Have a blood disorder, like sickle cell disease
• Have a body mass index (BMI) greater than 30 (have obesity)
• Are under 19 years old and take aspirin regularly
• Live in a long-term care facility or nursing home

These groups have the highest risk of complications that may require hospitalization and can be life-threatening.^[3]

How influenza is diagnosed

To diagnose the flu, your healthcare provider will do a physical exam, ask about your symptoms, and possibly order a test that detects flu viruses.^[10]

During times when flu is widespread in your community, you may not need a flu test if you have typical flu symptoms. However, a test may be helpful to guide your care or to know if you could spread the virus to others.^[10]

There are different types of flu tests available:^[10]

• Molecular tests: These tests look for genetic material from the flu virus. PCR tests (short for polymerase chain reaction) are one type of molecular test.
• Antigen tests: These tests look for viral proteins called antigens. Rapid influenza diagnostic tests are one example.

A flu test may be done by a pharmacy, in your healthcare provider’s office, or in the hospital. For people age 2 and older, at-home tests may be available.^[10]

It’s possible to have tests that check for both flu and other respiratory illnesses, such as COVID-19. You may have both flu and COVID-19 at the same time.^[10]

Treatment options

Most people with flu have mild illness and do not need medical care or prescription medicines. If you get sick with flu symptoms, in most cases, you should stay home and avoid contact with other people except to get medical care.^[9][20]

However, if you have flu symptoms and are at higher risk of serious complications, are very sick with flu, or are worried about your illness, contact your healthcare provider right away.^[9]

Antiviral medications

Antiviral drugs are prescription medicines that fight against flu viruses in your body. They are different from antibiotics, which fight bacterial infections and do not work against viruses like influenza.^[9][13]

Antiviral drugs can make your illness milder and shorten the time you are sick by about a day. They may also prevent some flu complications, like pneumonia. Treatment works best when started within 1 to 2 days after flu symptoms begin, but starting them later can still be helpful, especially if you are very sick.^[9][13]

There are four FDA-approved antiviral drugs recommended to treat flu:^[13]

• Oseltamivir phosphate (available as a generic or under the brand name Tamiflu), taken as a pill or liquid
• Zanamivir (brand name Relenza), taken as an inhaled powder
• Peramivir (brand name Rapivab), given intravenously (through a vein)
• Baloxavir marboxil (brand name Xofluza), taken as a pill

Healthcare professionals can determine if antiviral treatment is needed and which drug is most appropriate for you.^[12]

Prevention and vaccination

The best way to prevent the flu is by getting a flu vaccine every year. The first and most important step in preventing flu is to get vaccinated each year.^[1][5]

Annual flu vaccination

Everyone 6 months of age and older should get a flu vaccine every year, preferably before the onset of flu activity in the community. The best time to get vaccinated is in the fall, before flu season starts (typically October through May in North America), but getting vaccinated any time during flu season is still beneficial.^[1][5]

The flu vaccine is especially important for people at higher risk of serious flu complications. It also protects people around you, including those who are more vulnerable to serious illness.^[1][5]

Each year, the flu vaccine is updated to protect against the strains most likely to cause infection during the upcoming flu season. Although the vaccine is not 100% effective, it greatly lowers your chances of having severe complications from the flu. Even when flu vaccination does not prevent infection completely, it can make the illness milder and prevent serious complications.^[2][5]

The flu vaccine cannot give you the flu because it does not contain any live virus. Some people may get a mild fever or have discomfort for a short time after being vaccinated, but this is a sign that your body is responding to the vaccine, not the flu itself.^[7]

Everyday preventive actions

In addition to getting vaccinated, you can take these steps to help prevent infection with the flu:^[18]

• Avoid close contact with people who are sick
• Wash your hands often with soap and water, or use an alcohol-based hand sanitizer
• Avoid touching your eyes, nose, and mouth
• Cover your mouth and nose with a tissue when you cough or sneeze. If you don’t have a tissue, use the upper part of your sleeve, not your hands
• Clean and disinfect frequently touched surfaces at home, work, and school
• Get plenty of sleep, be physically active, manage your stress, drink plenty of fluids, and eat nutritious food

What to do if you get sick

If you develop flu symptoms, here’s what you should do:^[20][21]

Stay home and rest

You can go back to your normal activities when, for at least 24 hours, both of these are true: your symptoms are getting better overall, and you have not had a fever without using fever-reducing medicine. Stay home during this time to avoid spreading the flu to others.^[20]

Contact your healthcare provider

If you are at higher risk for complications, contact your healthcare provider as soon as flu symptoms begin. Your provider may prescribe antiviral drugs that work best when started within 2 days of becoming sick.^[20][21]

Seek emergency care if needed

You should go to the emergency room immediately if you have emergency warning signs such as:^[21]

• High or prolonged fever (above 101-102°F or lasting more than 3-5 days)
• Difficulty breathing or shortness of breath
• Pain or pressure in the chest
• Fainting or near-fainting
• Confusion
• Severe or persistent vomiting and inability to keep down food or water

Self-care at home

To help yourself feel better while recovering from the flu:^[23][25]

• Get plenty of rest and sleep
• Drink plenty of fluids such as water, warm soup, and other liquids to prevent dehydration
• Take over-the-counter pain medicine if needed, such as acetaminophen or ibuprofen, to lower fever and relieve headache and muscle aches (never give aspirin to anyone younger than 18, as it has been linked to a serious illness called Reye syndrome)
• Use a humidifier or breathe in steam to ease breathing
• Suck on cough drops or hard candy to soothe a sore throat
• Raise your head at night with an extra pillow if coughing keeps you awake

Most people recover from fever and other symptoms within a week without requiring medical attention, although malaise may persist for weeks. The cough can be severe and can last 2 weeks or more.^[4]

Prevent spreading the flu to others

To avoid spreading the flu:^[18][25]

• Stay home from work, school, and other public places until your fever has been gone for at least 24 hours without medicine
• Wash your hands regularly and keep your hands away from your face
• Cover your mouth when you cough or sneeze
• Wearing a mask can help protect others from breathing in infectious particles