What causes Flu?

What is flu?

Influenza, or the flu, is a viral infection that attacks the lungs, nose, and throat. It’s a contagious respiratory illness with symptoms ranging from mild to severe.

The flu and the common cold have similar symptoms. It can be difficult to distinguish between the two illnesses. In most cases, flu symptoms are more severe and last longer than the common cold.

To protect yourself against the flu, it’s best to understand how the virus spreads. The flu is highly contagious. It can spread quickly in households, schools, offices, and among groups of friends.

Anyone can become sick with the flu, but some people have a higher risk for infection. This includes children under the age of 5 and adults ages 65 and older.

The risk of the flu also increases if you have a weakened immune system or a chronic condition, such as:

  • heart disease
  • kidney disease
  • diabetes type 1 or 2

What are the symptoms of the flu?

In the beginning, the flu can mimic a common cold. Early symptoms may include:

  • sore throat
  • sneezing
  • runny nose

Symptoms often worsen as the virus progresses and can include:

  • fever
  • achy muscles
  • body chills
  • sweating
  • headache
  • dry cough
  • nasal congestion
  • fatigue
  • weakness

The flu doesn’t usually require a doctor’s visit. Symptoms often improve with home treatment in about a week. You can relieve symptoms with over-the-counter (OTC) cold and flu medications. It’s also important to get lots of rest and drink plenty of fluids.

However, some people are more likely to develop complications from the flu. If you or your child is in one of these high-risk groups, seek medical attention as soon as you suspect the flu.

High-risk groups include those who are:

  • under 2 years old
  • 65 years or older
  • pregnant or have recently given birth
  • have a chronic condition, such as diabetes, asthma, heart disease, or HIV

Complications of the flu

Most people recover from the flu without complications. But sometimes a secondary infection can develop, such as:

  • pneumonia
  • bronchitis
  • ear infection

If your symptoms go away and then come back a few days later, you may have a secondary infection. See a doctor if you suspect a secondary infection.

If left untreated, pneumonia can be life threatening.

How does the flu spread?

To protect yourself against the flu, it’s best to understand how the virus spreads. The flu is highly contagious. It can spread quickly in households, schools, offices, and among groups of friends.

According to the Centers for Disease Control and Prevention (CDC)Trusted Source, it’s possible to transmit the flu to someone as early as 1 day before symptoms begin and up to 5 to 7 days after you become sick.

After coming in contact with the virus, you’ll begin to show symptoms within 1 to 4 days. You can even transmit the virus to someone before you realize you’re sick.

The flu is primarily transmitted from person to person. If someone with the flu sneezes, coughs, or talks, droplets from them become airborne. If these droplets come in contact with your nose or mouth, you can become sick too.

You can also contract the flu from handshakes, hugs, and touching surfaces or objects contaminated with the virus. This is why you shouldn’t share utensils or drinking glasses with anyone, especially someone who might be sick.

How many types of flu viruses are there?

There are three different types of flu viruses that affect humans: type A, type B, and type C. (There’s a fourth, type D, that doesn’t affect humans.)

Animals and humans can contract type A flu because the flu virus can be transmitted from animals to humans. This virus constantly changes and can cause annual flu epidemics.

Type B flu can also cause seasonal outbreaks during the winter months. However, this type is typically less severe than type A and causes milder symptoms. Occasionally, type B can cause severe complications. Type B can only be transmitted from humans to humans.

Different strains cause type A and B flu.

Type C flu affects humans and some animals too. It causes mild symptoms and few complications.

How can the flu be prevented?

It’s important to protect yourself and your family from the virus because of the potential for complications.

Since the flu virus can be transmitted from person to person, make sure you wash your hands frequently with soap or use alcohol-based hand sanitizer. Also avoid touching your nose and mouth with unwashed hands.

The flu virus can live on hard surfaces and objects for up to 48 hoursTrusted Source. Use disinfectant wipes or spray on commonly touched surfaces in your home or at work to further protect yourself.

If you’re caring for someone who has the flu, wear a face mask to protect yourself. You can help stop the spread of the flu by covering your cough and sneezes. It’s best to cough or sneeze into your elbow instead of your hands.

Additionally, consider getting an annual flu vaccination. The vaccine is recommended for everyone over the age of 6 months. It protects against common strains of the flu virus.

Although the vaccine isn’t 100 percent effective, it can reduce the risk of flu by 40 to 60 percentTrusted Source, according to the CDC.

The flu vaccine is administered by injection in the arm. There’s also a nasal spray flu vaccine option for nonpregnant individuals between the ages of 2 and 49.


What you can do about the flu:

  • Get a flu shot. This will help protect you from life threatening complications such as pneumonia.
  • It takes 2 weeks for your body to make flu antibodies after you receive a vaccination. The earlier you get a flu vaccine, the better.
  • If you have an egg allergy, you can still get vaccinated. For people with severe egg allergy, the CDCTrusted Source recommends vaccination in a medical setting that can treat allergic reactions. Some forms of the vaccine may contain trace amounts of egg protein, but allergic reaction is unlikely.
  • Wash your hands frequently.
  • Cough and sneeze into your elbow.
  • Wipe down frequently touched surfaces in your home and office.


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  • Li JTC. (2019). Can I get the flu vaccine if I’m allergic to eggs?
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  • Vaccine effectiveness: How well do the flu vaccines work? (2020).

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