Insights on Influenza Blog
Mission: To save lives and reduce hospitalizations by protecting children, families, and communities against influenza.


What is Flu?

Influenza or “flu” is a highly contagious viral infection of the respiratory tract (nose, throat and lungs) that can cause mild to severe illness, and can result in complications that can lead to hospitalization or death. 

Brittney Peters, complications from flu


There are four different types of flu viruses - A, B, C, and D:

Influenza A: Influenza A viruses can be found in many different animals, including birds and mammals, as well as humans. These viruses are characterized by surface proteins, including hemagglutinin (“H”) and neuraminidase (“N”). These viruses are further separated into subtypes by number. There are 18 different H subtypes and 11 different N subtypes, and 131 subtype combinations have been detected in nature. Examples of flu viruses in this category include H1N1 and H3N2.

Influenza B: These viruses only affect people and are characterized according to where the virus was originally isolated. The two main categories (lineages) of influenza B include B/Yamagata and B/Victoria. 

Influenza C: This type of flu can affect people, but most often only causes mild illness. Influenza C infections are not thought to cause human flu epidemics, which are defined as widespread flu illness in a localized area. 

Influenza D: This type of flu primarily affects cattle and is not known to affect people. 


The flu can be serious, even deadly, for anyone – regardless of age or health status.

Everyone is at risk for being infected with the influenza virus, and anyone can spread the disease to others. 

Each flu season, between 5 and 20 percent of the U.S. population is infected with the flu virus, and approximately 8 percent experience flu illness.1

How is flu spread?

Flu spreads from person-to-person when people cough, sneeze, talk, or when a person touches a surface or object that has the flu virus on it and then touches their own mouth or nose.

Flu is not just a bad cold.

The flu is often confused with the common cold, but flu symptoms tend to develop quickly - usually one to four days after a person is exposed to the flu virus. Symptoms are usually more severe than the typical sneezing and congestion associated with the common cold.

cold symptoms vs flu symptoms
Austin Booth, lost to flu

Austin’s mother encourages all parents to teach their children everything they can do to help prevent spreading germs. Most importantly, she encourages everyone to get vaccinated against the flu every year.

Read Austin's Story

What are common flu symptoms?* 

Flu Symptom, fever


Flu Symptom, Headache


Flu Symptom, Fatigue Extreme tiredness

Fatigue or Extreme Tiredness

Flu Symptom, Muscle or body aches

Muscle or Body Aches

Flu Symptom, Dry cough

Dry Cough

Flu Symptom, Sore throat

Sore Throat

Flu Symptom, Runny or stuffy nose

Runny or Stuffy Nose

*Nausea, vomiting and diarrhea are symptoms more common in children.

It’s important to know when to seek medical attention for someone who is suffering with flu. If you have a child suffering with flu, find out when you should seek medical attention here. 

Flu kills more Americans every year than any other vaccine-preventable disease.

Influenza kills more Americans every year than any other vaccine-preventable disease.

How serious is the flu?

Each year in the U.S., the CDC estimates that influenza results in:

  • 9.3 – 45.0 million illnesses
  • 140,000 - 810,000 hospitalizations (about 20,000 of those hospitalized are children under the age of 5)
  • 12,000 - 61,000 deaths (including approximately 100 children)

While most people who get flu will recover in a few days to less than two weeks, some people will develop complications as a result of flu which can be life-threatening and even result in death.

Annual effects of flu in U.S.
Brittney Peters, complications from flu

Brittney passed away when her flu infection led to cerebral herniation, cardiac arrest, and respiratory distress syndrome.

Read Brittney's Story

What are complications of flu?

Some serious complications from flu include: 

  • Pneumonia 
  • Bacterial infections 
  • Inflammation of the heart (myocarditis), brain (encephalitis) or muscle (myositis, rhabdomyolysis) tissues
  • Multi-organ failure such as respiratory and kidney failure 
  • Sepsis, the body’s life-threatening inflammatory response to infection (click here to learn more about flu and sepsis)
  • Worsening of certain chronic conditions such as asthma, chronic heart disease or other conditions. 

The most common complications of flu are pneumonia, sepsis, and acute kidney injury.2

While everyone is susceptible to flu, it is especially dangerous for the following people:

Children younger than 5, but especially those younger than 2

People of all ages with certain medical conditions (listed below)*

*Certain medical conditions including asthma, chronic lung disease, heart disease, history of stroke, blood disorders; endocrine disorders (such as diabetes), kidney disorders, liver disorders, metabolic disorders, weakened immune system due to disease or medication (such as people living with HIV or AIDS, or cancer, or those on chronic steroids), people younger than 19 years old on long-term aspirin therapy, and obese people with a BMI of 40 or more.

How long do influenza symptoms last?

How long do flu symptoms last?

In most adults, an uncomplicated illness with flu can last about three to seven days, with periods of limited activity and bed rest. However, some symptoms such as a cough may linger as long as two weeks. Flu-related complications can also prolong the illness.3

When is someone contagious with flu?

An individual is typically contagious 24 hours before flu symptoms develop, which means that people are often unknowingly spreading illness before they even realize they are sick. 

They remain infectious for up to five to seven days after symptoms first appear. So even if an individual is feeling better, they can still be spreading the influenza virus to others. It is also believed that children can remain infectious longer, until all their symptoms resolve.

People with flu are no longer contagious after 24 hours without a fever and without the use of fever-reducing medications.

How long is influenza contagious?

When is flu “season”?

Flu is continually circulating somewhere in the world all throughout the year. However, flu activity in the U.S. generally occurs from October through May, with peak activity typically occurring sometime between December and February.  

A special life-support machine was needed as the virus began to attack Breanne’s heart and brain stem.

Breanne required a special life-support machine as the flu virus began to attack her heart and brain stem.

Read Breanne's Story

Visit these other pages to learn more:

Page last reviewed: July 2020. 

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