What Is Flu? Symptoms, Transmission, Treatment and Flu Vaccine

Created: 24.07.2026  ·  Last Updated: 24.07.2026  ·  Category: Internal Medicine  ·  Prepared by the Academic Hospital Web and Medical Editorial Board.

What Is Flu?

Flu, medically known as influenza, is a contagious respiratory illness caused by influenza viruses. Flu is common especially in autumn and winter and can spread easily through coughing, sneezing or close contact.

Important Information: Flu can be more severe than the common cold, and complication risk is higher especially in older adults, pregnancy, chronic diseases, immunosuppression and young children. Medical evaluation should not be delayed in severe shortness of breath, chest pain, confusion or prolonged high fever.

What Are the Symptoms of Flu?

Although it may be confused with the common cold, flu can be more severe. Flu symptoms usually begin suddenly and worsen within a few days.

  • High fever
  • Severe weakness and fatigue
  • Muscle and joint pain
  • Headache
  • Runny or stuffy nose
  • Sore throat
  • Dry cough

You can contact us for evaluation if flu symptoms, high fever, severe fatigue or risk factors are present.

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What Is the Difference Between Flu and the Common Cold?

Flu (influenza) and the common cold are often confused, but they are different illnesses.

Common Cold Flu
Starts gradually Starts suddenly
Fever is rare or may be milder High fever may occur
Muscle and joint pain is usually mild Muscle and joint pain may be more severe
Fatigue may be short-lasting Intense and prolonged fatigue may occur

How Does Flu Spread?

Flu generally spreads in two main ways: through droplets and contact. It can spread through microscopic virus-containing droplets released when a sick person coughs, sneezes or talks. These droplets may reach the mouth, nose or eyes of a nearby healthy person and enter the body through the respiratory tract. The risk of transmission increases in crowded, closed and poorly ventilated environments.

Surfaces such as door handles, desks, phones and cups touched by a sick person after coughing or sneezing may also pose a transmission risk. If a healthy person touches these surfaces and then touches their mouth, nose or eyes without washing their hands, the virus may spread. Close contact such as shaking hands, kissing or speaking at close distance may also transmit the virus.

How Long Is Flu Contagious?

Flu may be contagious starting 1 day before symptoms appear and generally until days 5 to 7 of illness. This means a person may spread the virus before realizing they are sick. In children and immunocompromised people, the contagious period may be longer.

How Is Flu Treated?

Flu is a treatable illness. In addition to medications recommended under physician supervision, healthy nutrition, plenty of fluids and adequate rest help support the recovery process.

In people at higher risk or in severe flu, a physician may consider antiviral medications. Antiviral treatment may be more effective when started early after symptom onset.

Flu and Antibiotic Use

Flu is a viral illness; therefore, antibiotics do not work against influenza viruses. Antibiotics should only be used for bacterial infections and with physician recommendation. Unnecessary antibiotic use may be harmful because of antibiotic resistance and side effects.

Why Is the Flu Vaccine Important?

The flu vaccine is one of the important methods for protection against influenza. Because circulating flu viruses may change each year, vaccine composition is updated seasonally. The vaccine helps the immune system develop a response against the virus. A vaccinated person may still get flu, but vaccination may help reduce the risk of severe illness, hospitalization and complications.

Immune response usually develops within about two weeks after vaccination. The timing, suitability and risk groups for flu vaccination should be planned according to age, health status, pregnancy status and physician evaluation.

Who Should Get a Flu Vaccine?

Flu vaccination is especially important for the following groups:

  • People aged 65 and older
  • People with chronic diseases
  • Pregnant people
  • Healthcare workers
  • People with weakened immune systems
  • Children, especially ages 6 months to 5 years

Flu is a serious respiratory illness that affects millions of people every year. With correct treatment, rest and preventive methods, it is possible to reduce the risk of flu. Flu vaccination is an important preventive method for reducing the risk of severe disease.

Evaluation Without Delay in Flu Symptoms

Physician evaluation is important if high fever, shortness of breath, severe fatigue, chronic disease or pregnancy-related risk factors are present.

Frequently Asked Questions

What is flu?
Flu, or influenza, is a contagious respiratory illness caused by influenza viruses. It can affect the nose, throat and lungs.
Are antibiotics used for flu?
Flu is a viral illness; antibiotics do not work against influenza viruses. Antibiotics should only be used if a physician suspects a bacterial infection.
How long is flu contagious?
Flu may be contagious from one day before symptoms begin and generally during the first 5-7 days of illness. This period may vary in some people.
Who should consider flu vaccination?
Flu vaccination is especially important for people aged 65 and older, people with chronic diseases, pregnant people, healthcare workers, immunocompromised people and young children.
Academic Hospital note: Flu evaluation should consider age, pregnancy, chronic disease, immune status, symptom severity, fever duration and signs of complications together. You can book an appointment for evaluation.

References

  1. CDC. About Influenza
  2. CDC. Signs and Symptoms of Flu
  3. CDC. How Flu Spreads
  4. CDC. Treatment of Flu
  5. CDC. Seasonal Flu Vaccine Basics
  6. NHS. Flu