What Is Influenza (Flu)?

Influenza, commonly known as the flu, is an acute viral infection affecting the upper and lower respiratory tract. In contrast to a common cold, flu appears suddenly and often begins with a high fever, severe body aches, and marked fatigue. The virus attacks the mucous membranes of the nose, throat, and lungs, causing widespread inflammation. Flu tends to spread rapidly during seasonal outbreaks, making it one of the most common and impactful respiratory infections worldwide.

Main Causes of Flu

The flu is caused by influenza viruses, primarily types A and B. The infection spreads through several routes:

• Airborne transmission: tiny droplets released during coughing or sneezing.
• Contact transmission: touching contaminated surfaces (door handles, phones, public transport).
• Close interaction: living with or being near an infected person.
• Seasonal outbreaks: most intense during fall and winter.

Additional factors increasing the risk of infection:

• weakened immune system,
• chronic respiratory diseases,
• crowded environments,
• sudden temperature changes.

Key Symptoms of Influenza

Flu symptoms usually appear abruptly within a few hours. Most patients can identify the exact moment they started feeling ill due to the sudden rise in fever, chills, and joint pain. The body attempts to fight the virus by activating the immune response, which leads to inflammation across the respiratory system. This inflammation causes throat irritation, dry cough, headaches, and pronounced weakness, often accompanied by a complete loss of appetite.

Typical Flu Symptoms (Full List)

• high fever (38–40°C),
• chills and sweating,
• muscle and joint pain,
• severe fatigue and weakness,
• dry cough (sometimes painful),
• sore throat,
• runny or stuffy nose,
• headaches,
• dizziness,
• nausea or diarrhea (more common in children),
• difficulty sleeping due to fever or cough,
• sensitivity to light and noise.

Stages of Flu Progression

Flu typically develops and resolves in predictable stages. During the first 24–48 hours, the body experiences the strongest viral activity, which is why fever and inflammation peak at this time. As the immune system begins fighting the infection, symptoms shift from acute fever to persistent cough and fatigue. The recovery period may last longer than the main illness due to the body’s need to regain strength and restore damaged mucous tissues.

Day-by-Day Flu Timeline

Day 1–2:

• sudden fever and chills,
• sharp muscle pain,
• dryness in throat,
• beginning of cough.

Day 3–4:

• strongest weakness,
• persistent headache,
• dry, irritating cough,
• reduced appetite.

Day 5–7:

• fever decreases,
• cough remains,
• energy slowly returns.

After 7 days:

• lingering fatigue,
• slowly improving breathing,
• residual cough for up to 2 weeks.

When Flu Becomes Dangerous

Although most people recover without complications, influenza can become dangerous for certain groups. Children, elderly individuals, pregnant women, and patients with chronic diseases are at the highest risk. In these cases, flu can progress into pneumonia, severe dehydration, or cause worsening of existing health conditions. Recognizing dangerous symptoms early allows patients to seek medical care before complications develop into life-threatening conditions.

Warning Signs — When You Must See a Doctor

Seek medical help immediately if you experience:

• difficulty breathing or shortness of breath,
• chest pain or pressure,
• lips or face turning blue,
• confusion or sudden dizziness,
• inability to drink or severe dehydration,
• fever lasting more than 3 days,
• symptoms in children under 5 or adults over 65,
• worsening symptoms after initial improvement.

Possible complications:

• viral pneumonia,
• bacterial pneumonia,
• sinus infection,
• ear infection,
• bronchitis,
• severe dehydration.

Diagnosis of Influenza

Doctors usually diagnose flu based on symptoms alone, especially during seasonal outbreaks when the virus is widespread. In cases where symptoms are severe or complications are suspected, rapid antigen tests or PCR tests may be used to detect influenza viruses. These tests help differentiate flu from other respiratory infections that may require different treatments.

Treatment Options

Antiviral medications — effective during the first 48 hours:

• reduce duration of illness,
• decrease risk of complications.

Fever control:

• acetaminophen or ibuprofen,
• cool compresses,
• adequate hydration.

Cough relief:

• warm fluids,
• honey (for adults),
• humidifiers,
• throat lozenges.

Important:

• antibiotics do not treat flu,
• rest and hydration are essential,
• avoid physical activity.

Home Care & Recovery

For most people, flu is best managed at home. Drinking warm fluids, maintaining proper room humidity, and resting help the body recover faster. Sleep plays a key role in restoring the immune system, while regular hydration prevents dehydration caused by fever. It is recommended to avoid alcohol, smoking, and intense physical activity during recovery.

Prevention

• yearly flu vaccination,
• frequent handwashing,
• avoiding close contact with sick people,
• wearing masks during outbreaks,
• disinfecting frequently touched surfaces,
• boosting immunity with adequate sleep and nutrition.

Summary

Influenza is a widespread viral infection that causes sudden fever, cough, and profound weakness. Most people recover within a week, but for some groups, the illness can become dangerous. Early recognition of symptoms, timely treatment, proper home care, and annual vaccination greatly reduce the risk of complications and help maintain overall health during flu season.

Is flu contagious?

Yes. It spreads easily through air and close contact.

How long does flu last?

Normally 5–7 days, but weakness may last up to 2 weeks.

Can I treat flu without seeing a doctor?

Yes, if symptoms are mild. But seek help if fever persists or symptoms worsen.

Do I need antibiotics for flu?

No. Flu is viral, and antibiotics don’t help unless a bacterial complication develops.

When is flu most dangerous?

For children, elderly people, pregnant women, and patients with chronic diseases.