Causes and Risk Factors of the Flu

The flu is caused by the influenza virus, which is spread from person to person via respiratory droplets. These may be shared due to sneezing, coughing, talking, or blowing one's nose, or contact with surfaces contaminated by these droplets. It is possible to get the flu if you've had it before, as there are different strains and they are constantly mutating—meaning your immunity can never be completely ahead of the game.

The best thing you can do to prevent yourself from being infected with the flu virus is to get your flu shot each year before the start of the flu season. You can also lower your chances by avoiding close contact with sick people and being aware of other factors that could put you at risk of serious complications.


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Common Causes

The influenza virus has several types, subtypes, and strains. Usually, only one or two strains are circulating during the annual influenza season. In the U.S., the flu season is October through April.

Julie Bang / Verywell

Infectious Period

It's possible to spread the flu one day before you experience symptoms until five to seven days after you begin to feel sick. Young children and people with compromised immune systems may be at higher risk of infecting others for longer periods of time.

Droplet Transmission

The influenza virus infects the nose, lungs, and throat. It spreads when an infected person coughs, sneezes, or talks in the presence of other people. Droplets containing the virus may come into contact with a person via their mouth, nose, or eyes, and then cause infection.

According to the Centers for Disease Control and Prevention (CDC), the best evidence is that influenza is usually spread by large droplet transmission, which can occur within six feet of an individual.

Surface Transmission

Flu viruses can live on surfaces for a few hours. A 2011 study found viruses still alive after four hours on most surfaces, and up to nine hours on some non-porous surfaces, yet all were gone after 24 hours.

Touching a surface and then touching your mouth, nose, or eyes may transmit the flu. The virus may end up on a surface due to respiratory droplets or hands contaminated by respiratory secretions. Social interactions such as shaking hands can transmit the virus in this way as well.

Handwashing can break the transmission cycle so you aren't picking up germs from surfaces or depositing them. Washing your hands with soap and water for 20 seconds and using alcohol-based hand sanitizer when you don't have access to soap and water are key parts of preventing the spread of influenza.

Health and Age Risk Factors

Anyone is at risk of becoming infected with influenza and spreading it to other people. However, certain groups are more at risk of catching influenza because their immune systems don't produce enough protective antibodies when exposed to the vaccine. These groups include:

  • Young children
  • Adults aged 65 years and older
  • Those with pre-existing conditions including cardiovascular disease, diabetes, chronic respiratory failure, or pregnancy
  • Those in immunocompromised states, such as people with blood cancer or HIV/AIDS, who are undergoing cancer treatment, or who are taking other medications that suppress the immune system

These groups are also at higher risk of experiencing flu-related complications, as are people with asthma or heart disorders, who may have a worsening of those conditions.

While all children younger than 5 years old are considered to be at high risk for serious flu complications, the highest risk is for those younger than 2 years old. The highest hospitalization and death rates are among infants younger than 6 months old. (Note: those under 6 months cannot be vaccinated against the flu.)

Lifestyle Risk Factors

Your risk of catching the flu and spreading it to others is raised by certain habits and practices.

Failure to Get the Annual Flu Shot

The best way to reduce your risk of the flu is by getting the annual influenza vaccine, which is recommended for all people six months and older. If you choose not to get the vaccine, you are at risk of catching influenza and transmitting it to vulnerable people around you.

The flu shot is reformulated each year based on the prediction of which strains of flu will be circulating. While the prediction is not always perfect, it can prevent catching the flu or may make the flu milder if you do catch it.

The vaccine works by stimulating your body to create antibodies that will fight the influenza virus. If you are actually exposed to the influenza virus, your body will recognize it and be able to fight it off.

Exposure to More People

Some are more at risk because their circumstances expose them to larger groups of people or those more likely to have influenza. These include those who work in daycare centers, retail, and healthcare settings.

Using public transportation or congregating in crowded places for entertainment can also increase your expose you to people who can transmit influenza. As well, some workplace designs crowd workers together or encourage shared surfaces that may increase the risk of transmission.

Lax Hygiene Practices

As the influenza virus can be picked up from surfaces, failure to wash your hands after using the restroom, before eating, or before touching your face increases the risk of transmission. Not covering coughs and sneezes can result in droplet transmission to others.

Poor Lifestyle Habits

You may increase your risk of influenza if you don't maintain your overall health by getting enough sleep, being physically active, eating nutritiously, drinking enough fluids, and managing your stress. Healthy habits may help prevent or manage conditions that raise your risk of complications from influenza. These include type 2 diabetes and obesity.

A Word From Verywell

The best way to reduce your chances of getting the flu is by getting your flu vaccine. By additionally being aware of increased risk factors and taking steps to avoid spreading germs and coming into contact with sick people, you can further protect yourself from becoming infected this year.

Frequently Asked Questions

  • What causes the flu?

    The flu is caused by influenza viruses. There are three types that cause disease in humans: influenza A virus (IAV), influenza B virus (IBV), and influenza C (ICV). Of these, IAV and IBV account for the vast majority of infections. While IAV and IBV are both associated with seasonal flu, IAV tends to be more serious and is the only type that causes flu pandemics.

  • How many different flu strains are there?

    There are thought to be over 110 different subtypes of influenza A virus, including avian flu and swine flu. Because influenza B virus mutates at a far slower rate, there are fewer genetic variations, and the virus is instead classified into two lineages: B/Yamagata and B/Victoria.

  • How is flu spread?

    Influenza is mainly spread via respiratory droplets and possibly through smaller airborne particles. Transmission can therefore occur as a result of breathing, talking, cough, or sneezing. The virus is less commonly spread by person-to-person contact or contact with contaminated surfaces or objects (fomites).

  • How long can influenza survive outside the body?

    On non-porous surfaces, influenza viruses can survive for hours and potentially be spread by hand-to-face contact. Regularly washing your hands and cleaning surfaces (as well as avoiding touching your face) can reduce the risk of transmission if someone in the house is ill.

  • Who is at the highest risk of flu complications?

    Anyone can get flu, but certain groups are at a higher risk of flu complications like bacterial pneumonia. These include:

    • Children under age 5
    • Adults 65 and over
    • Pregnant people
    • Immunocompromised people
    • People with obesity
    • Persons with chronic health conditions such as asthma, COPD, heart disease, liver disease, or kidney disease
10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.