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What Causes the Flu?

Learn about the different types of flu viruses and how to prevent getting them.

Influenza, commonly known as the flu, is a viral respiratory infection caused by the influenza viruses. It can affect your entire respiratory system—nose, throat and lungs.1

The flu is very contagious and can spread easily. The viruses usually spread through droplets that travel through the air when someone with the flu coughs or sneezes. These droplets can be inhaled by anyone who happens to be nearby or can be picked up by touching a surface that has the germs, like a desk, door handle or keyboard—and then transferred by touching your mouth, eyes or nose.4

Because flu viruses change every year, researchers worldwide monitor the 3 or 4 strains that are most likely to infect the most people. The researchers then create a seasonal flu vaccine to best meet the needs of the current cold and flu season.2

Different Types of Flu

There are four different types of influenza viruses that exist.5 Some of these flu viruses have their own subtypes, lineages, groups, and sub-groups.5 The types of flu viruses break down into type A, type B, type C, and type D.5

Influenza A

Influenza A is the only type of flu viruses that can cause global flu epidemics when they spread.5 Influenza A is usually seasonal and occurs almost every winter in the United States.5 This type has two subtypes: H1N1 and H3N2.5 Each of those subtypes break further into different groups and sub-groups.

Influenza B

Influenza B is also a seasonal flu virus that typically occurs every winter season in the United States.5 Instead of subtypes, this type of virus breaks down into two lineages: B(Victoria) and B(Yamagata).5 Those lineages then break down further into different groups and subgroups.

Influenza C

While humans can contract Influenza C, it tends to be a mild version of the virus. Because of its mildness, it has a low likelihood to cause a flu epidemic.5

Influenza D

Influenza D is the fourth type of influenza virus. However, humans don’t contract this type of flu virus.5 It usually only affects different types of animals.5

It is important to check with your local and statewide healthcare providers about which flu viruses are spreading in your area because the virus is constantly changing.6 There are two ways flu viruses can change throughout the year:

1. Antigenic Drift

This is when the existing flu viruses mutate and change the number of surface proteins the virus has.6 This drift isn’t an immediate change because the viruses mutate over time as they  replicate in more people.6 This type of change is the main reason why people can get the flu more than once.6 Most of the seasonal flu shot vaccines and nasal spray vaccines target this type of flu virus change.6

2. Antigenic Shift

While the antigenic drift occurs over a period of time, the antigenic shift happens suddenly. This type of change occurs  when flu viruses abruptly get new proteins in their surface.6 Something that can cause a sudden, but major, change in the flu viruses is when a flu virus from a type of animal becomes contagious to humans.6 This type of change occurs rarely, and there have only been four pandemics in the past century due to this type of change in the flu virus.6

You can check with your local healthcare provider or the Centers for Disease Control and Prevention for any updates on changes to existing flu viruses, what types of changes are occurring, and the different flu vaccines that are being offered to offset those changes.

Flu Symptoms

The flu can range from mild to severe—and symptoms usually develop quickly.4 In fact, one of the main differences between a cold and the flu is that the flu usually comes on fast. Even though a cold and the flu share a lot of the same symptoms, people with the flu will usually feel a lot worse.


Chills, cold sweats and shivers

Aching joints and limbs

Feeling tired, exhausted, fatigued

A cough

Sore throat

Runny or stuffy nose


High-Risk Groups

There are some groups of people who are at higher risks of contracting a flu virus during the year. Some of these groups are:7

  • Adults that are at 65 years of age and older7
  • Children that are younger than 2 years old7
  • People who have blood disorders (such as sickle cell disease)
  • People who have heart diseases (such as congenital heart disease, congestive heart failure and coronary artery disease)
  • People with chronic lung disease (such as chronic obstructive pulmonary disease [COPD] and cystic fibrosis)
  • People who have kidney diseases
  • People who are obese with a body mass index [BMI] of 40 or higher.
  • People with a weakened immune system from diseases (such as HIV or AIDS, or some cancers such as leukemia) or medications (such as chemotherapy or radiation)
  • People who have had a stroke
  • Pregnant women up to 2 weeks after the end of pregnancy

How to Help Prevent the Flu

Getting an annual flu vaccine before the start of flu season is the first and most important step in protecting against the flu. Since these viruses constantly change, even if you’ve had the flu in the past, your body might be susceptible to a new strain on any given year.3 That’s why it’s important to get a flu vaccine every year—that way you’ll be best prepared for the most likely flu strains that will be going around during the year.

There are also some everyday steps you can take to help prevent coming into contact with—and spreading—the germs that cause the flu:3

• Wash your hands often. Thorough hand-washing is an effective way to prevent many common infections.
• Cover your coughs and sneezes. Cover your mouth and nose with a tissue when you sneeze or cough to help prevent spreading the virus.
• Stay home and rest. If you're sick, stay home for at least 24 hours after your fever subsides so that you lessen your chance of infecting others.
• Stay on top of the flu. Use our cold & flu tracker to get a thorough report of cold and flu incidences in your area—so you’re able to prepare and be ready.

If you come down with the flu, over-the-counter cold and flu medicines like Theraflu can help ease your most severe symptoms and get you back on your feet. And with powerful symptom relief, especially at night, you’ll be able to rest easier as you recover—and feel better.

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