Influenza, commonly known as the flu, is a contagious viral infection that affects the respiratory tract. Symptoms such as fever, sore throat, and body aches typically resolve on their own within a week. While most people recover without complications, the flu can be more serious for certain groups. Flu season generally runs from October through May, and nearly one in five people in the United States contract the flu each year. Although there’s no cure, getting a yearly flu shot can reduce your risk of infection.

Types of Influenza

While many people refer to "the flu" as a single illness, there are actually hundreds of different strains. Flu viruses constantly evolve to escape immune detection and spread to new hosts. There are four main types of influenza viruses: A, B, C, and D. Only types A and B are responsible for the annual flu epidemics. Type C causes mild illness, and type D is not known to infect humans.

Influenza A

Influenza A includes subtypes such as H1N1 and H3N2, which are typically included in the annual flu vaccine. This type has been responsible for past global flu pandemics.

Influenza B

Influenza B generally causes milder illness than type A. A single strain of influenza B is also included in the yearly flu shot.

Influenza C

This type causes very mild symptoms or no symptoms at all and is far less common than A or B.

Influenza D

This strain only affects cattle and has not been found to infect humans.

Flu Symptoms

Flu symptoms often appear suddenly and typically develop two to three days after infection. Common symptoms include:

  • Body aches and headaches

  • Dizziness

  • Dry cough

  • Fatigue

  • Fever (typically between 102°F and 106°F)

  • Chills

  • Flushed face

  • Loss of appetite

  • Nausea or vomiting

  • Runny nose and sneezing

  • Sore throat

Symptoms like fatigue or a persistent cough may linger for weeks. Some people also experience difficulty breathing, especially those with asthma or chronic health conditions.

Causes of Influenza

The flu causes inflammation in the upper respiratory tract and spreads through tiny droplets released when an infected person coughs, sneezes, laughs, or talks. You can get the flu if these droplets enter your mouth or nose. A person is usually contagious one day before symptoms appear and remains contagious for up to a week after. In some people, especially children or those with weakened immune systems, this contagious period may last even longer. The flu virus can survive on surfaces for up to 48 hours, although surface transmission is less common than person-to-person spread.

Who Is at Risk?

Anyone can get the flu, but certain groups are at higher risk of complications. These include:

  • Children under 5 and adults over 65

  • People with asthma

  • Individuals with heart disease or diabetes

  • Pregnant people

Diagnosing the Flu

In most cases, people recover at home without needing a formal diagnosis. However, if you are at risk of complications or have severe symptoms, it's important to contact a healthcare provider. Diagnosis may be based on a physical exam and symptoms, especially during flu outbreaks. If needed, tests such as rapid influenza diagnostic tests (RIDTs) or molecular assays may be used. These tests analyze nasal or throat swabs to detect the virus.

Flu vs. Cold or COVID-19

Flu symptoms often begin abruptly, while cold symptoms develop more gradually. The flu and COVID-19 share many symptoms such as fever, cough, and body aches. However, COVID symptoms tend to appear over a longer incubation period. The only way to confirm COVID is through testing.

Treating the Flu

Most people do not need medication for the flu. The immune system usually clears the infection on its own. However, antiviral medications such as Tamiflu (oseltamivir) and Relenza (zanamivir) may be prescribed to shorten the duration of illness—especially if taken within 48 hours of symptom onset.

Home Remedies

If you have mild symptoms, you can manage them at home by:

  • Getting plenty of rest

  • Staying hydrated

  • Taking over-the-counter pain relievers like ibuprofen or acetaminophen

  • Avoiding alcohol and tobacco

  • Using cough drops or throat sprays to ease discomfort

Prevention

The best way to prevent the flu is by getting an annual flu vaccine. Experts update the vaccine each year based on the most common circulating strains. Although no vaccine is 100% effective, getting vaccinated boosts your immunity and reduces your chances of severe illness.

Other prevention tips include:

  • Washing your hands frequently

  • Avoiding close contact with sick individuals

  • Maintaining a healthy lifestyle

  • Avoiding touching your face

Complications of the Flu

While most people recover from the flu without complications, others may develop serious issues, such as:

  • Encephalitis – Inflammation of the brain

  • Myocarditis – Inflammation of the heart muscle

  • Myositis – Muscle inflammation, often caused by viral infection

  • Pneumonia – Lung infection caused by the virus spreading to the lungs

  • Rhabdomyolysis – Muscle breakdown leading to harmful proteins in the bloodstream

  • Sepsis – A potentially fatal immune response to infection

  • Sinus or ear infections – Often caused by swelling and mucus buildup during illness

When To See a Healthcare Provider

Most flu cases resolve within a week. You should contact your healthcare provider if:

  • Your symptoms don’t improve or get worse

  • You are at high risk for complications

  • Home remedies are not helping

When To Seek Emergency Care

Seek immediate medical attention if you or your child experiences:

  • Severe or persistent chest, muscle, or abdominal pain

  • Difficulty breathing or shortness of breath

  • Blue lips or face (especially in children)

  • Seizures

  • Confusion or trouble staying awake

  • Signs of dehydration

  • Fever that disappears and then returns

  • Rib retractions when breathing (in children)

Infants under 3 months with any fever or children with a fever above 104°F should be seen by a healthcare professional immediately.

A Quick Review

The flu is a common but contagious viral infection affecting the respiratory tract. Symptoms come on quickly and include fever, cough, and fatigue. Most people recover in a week, but high-risk individuals may face serious complications. You can help prevent the flu by getting a flu shot each year, practicing good hygiene, and staying home if you're sick.