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.
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.
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.
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.
What are common flu symptoms?*
Fatigue or Extreme Tiredness
Muscle or Body Aches
Runny or Stuffy Nose
*Nausea, vomiting and diarrhea are symptoms more common in children.
Influenza kills more Americans every year than any other vaccine-preventable disease.
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.
While everyone is susceptible to flu, it is especially dangerous for the following people:
*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.
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
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.
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.
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Page last reviewed: July 2020.