Flu (Influenza): Symptoms, Causes, Treatment & Prevention

What is Flu (Influenza)?

The flu, or influenza, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to hospitalization or even death. The flu is different from a cold—it usually comes on suddenly and may include fever, body aches, and fatigue.

According to the Centers for Disease Control and Prevention (CDC), flu seasons are unpredictable and can be severe. Each year, millions of people get the flu, hundreds of thousands are hospitalized, and thousands to tens of thousands die from flu-related causes.

Common Causes

The flu is caused by influenza viruses, which belong to three main types: A, B, and C. Type A and B viruses cause seasonal epidemics of respiratory illness almost every winter in the United States. Type C infections generally cause mild respiratory symptoms and are not thought to cause epidemics. The most common causes and risk factors include:

• Influenza A viruses: These are the most common and can infect humans and animals. Subtypes include H1N1 and H3N2.
• Influenza B viruses: These are found only in humans and can cause seasonal outbreaks.
• Close contact with infected individuals: The flu spreads mainly through droplets made when people with the flu cough, sneeze, or talk.
• Touching contaminated surfaces: You can get the flu by touching a surface or object with the virus on it and then touching your mouth, nose, or eyes.
• Weakened immune system: People with chronic illnesses, pregnant women, young children, and older adults are at higher risk.
• Crowded environments: Schools, nursing homes, and public transportation increase the risk of transmission.
• Not being vaccinated: The flu vaccine reduces the risk of illness, hospitalization, and death.
• Seasonal changes: Flu viruses circulate more during fall and winter months.
• Travel: Exposure to different strains of the virus in other regions or countries.
• Poor hygiene: Not washing hands regularly or covering coughs and sneezes.

For more details on how the flu spreads, visit the World Health Organization (WHO).

Associated Symptoms

Flu symptoms often come on suddenly and can include:

• Fever or feeling feverish/chills (not everyone with the flu will have a fever)
• Cough
• Sore throat
• Runny or stuffy nose
• Muscle or body aches
• Headaches
• Fatigue (tiredness)
• Vomiting and diarrhea (more common in children than adults)

Symptoms typically begin about 2 days after exposure to the virus and can last for about a week, though fatigue may persist for several weeks. Some people, especially children, may also experience nausea, vomiting, or diarrhea, but these symptoms are more common with gastroenteritis ("stomach flu"), which is not related to influenza.

For a comparison between flu and cold symptoms, refer to this Mayo Clinic resource.

When to See a Doctor

Most people with the flu can recover at home without medical care. However, certain groups are at higher risk of complications and should seek medical attention if they develop flu-like symptoms. These groups include:

• Children younger than 5 years old, especially those under 2
• Adults 65 years and older
• Pregnant women
• People with chronic medical conditions (e.g., asthma, diabetes, heart disease)
• People with weakened immune systems (e.g., due to HIV/AIDS, chemotherapy, or long-term steroid use)
• Residents of nursing homes or long-term care facilities
• People with a body mass index (BMI) of 40 or higher

If you are in one of these high-risk groups or your symptoms are severe or worsening, contact your healthcare provider. Antiviral medications may be prescribed to reduce the severity and duration of the flu, especially if started within 48 hours of symptom onset.

Diagnosis

Doctors typically diagnose the flu based on symptoms, especially during flu season. However, in some cases, they may use specific tests to confirm the diagnosis:

• Rapid influenza diagnostic tests (RIDTs): These can provide results in about 10-15 minutes but may not be as accurate as other tests.
• Rapid molecular assays: These tests detect genetic material from the virus and can provide results in 15-30 minutes.
• Viral culture: This test involves growing the virus in a lab and can take several days for results.
• Reverse transcription-polymerase chain reaction (RT-PCR): This is a highly accurate test that can identify the specific type and subtype of influenza virus.

Testing is particularly important for people at high risk of complications or those who are hospitalized with flu-like symptoms. For more information on flu testing, visit the CDC’s flu diagnosis page.

Treatment Options

Treatment for the flu focuses on relieving symptoms and preventing complications. Here are the main options:

Medical Treatments

• Antiviral medications: Prescription drugs like oseltamivir (Tamiflu), zanamivir (Relenza), or peramivir (Rapivab) can reduce the severity and duration of the flu if started within 48 hours of symptom onset. These are especially important for high-risk individuals.
• Over-the-counter (OTC) medications:
  • Pain relievers (e.g., acetaminophen, ibuprofen) for fever, headaches, and body aches.
  • Decongestants (e.g., pseudoephedrine) for nasal congestion.
  • Cough suppressants (e.g., dextromethorphan) or expectorants (e.g., guaifenesin) for cough.
• Hydration therapy: In severe cases, intravenous (IV) fluids may be needed to treat dehydration.

Home Remedies and Self-Care

• Rest: Stay home and get plenty of sleep to help your immune system fight the virus.
• Hydration: Drink plenty of fluids like water, herbal teas, broths, and electrolyte solutions to prevent dehydration.
• Humidifier: Use a humidifier to add moisture to the air and ease congestion and cough.
• Warm compresses: Apply to the forehead or sinuses to relieve headaches or sinus pressure.
• Gargling salt water: This can help soothe a sore throat.
• Honey: A spoonful of honey can help suppress coughs (do not give honey to children under 1 year old).

For more tips on managing flu symptoms at home, check out this Harvard Health guide.

Prevention Tips

Preventing the flu starts with good hygiene and vaccination. Here are key strategies to reduce your risk:

• Get vaccinated annually: The flu vaccine is updated each year to protect against the most common strains. The CDC recommends vaccination for everyone 6 months and older, especially high-risk groups. Vaccination can reduce flu illnesses, doctor visits, and missed work or school.
• Wash your hands: Use soap and water for at least 20 seconds, especially after coughing, sneezing, or being in public places. If soap isn’t available, use an alcohol-based hand sanitizer.
• Cover coughs and sneezes: Use a tissue or the inside of your elbow to prevent spreading germs.
• Avoid close contact: Stay away from people who are sick, and if you’re sick, stay home for at least 24 hours after your fever is gone (without fever-reducing medication).
• Clean and disinfect surfaces: Regularly clean frequently touched surfaces like doorknobs, phones, and countertops.
• Boost your immune system:
  • Eat a balanced diet rich in fruits, vegetables, and lean proteins.
  • Exercise regularly to support overall health.
  • Get enough sleep (7-9 hours for adults).
  • Manage stress through relaxation techniques like meditation or yoga.
• Avoid touching your face: This can reduce the risk of transferring viruses from your hands to your mouth, nose, or eyes.
• Consider antiviral medications: If you’ve been exposed to the flu, your doctor may prescribe antivirals as a preventive measure, especially if you’re at high risk.

For more on flu prevention, visit the National Institutes of Health (NIH).

Emergency Warning Signs

Seek immediate medical attention if you or someone you care for experiences any of the following emergency warning signs. These symptoms may indicate a severe case of the flu or complications like pneumonia:

In Children:

• Fast breathing or trouble breathing
• Bluish lips or face
• Ribs pulling in with each breath
• Chest pain
• Severe muscle pain (child refuses to walk)
• Dehydration (no urine for 8 hours, dry mouth, no tears when crying)
• Not alert or interacting when awake
• Seizures
• Fever above 104°F (40°C)
• Fever or cough that improves but then returns or worsens
• Worsening of chronic medical conditions

In Adults:

• Difficulty breathing or shortness of breath
• Persistent pain or pressure in the chest or abdomen
• Persistent dizziness, confusion, or inability to arouse
• Seizures
• Not urinating
• Severe muscle pain
• Severe weakness or unsteadiness
• Fever or cough that improves but then returns or worsens
• Worsening of chronic medical conditions

If you or a loved one experience any of these symptoms, go to the nearest emergency room or call 911 immediately. Early treatment can be life-saving.

For more information on flu complications, visit the Cleveland Clinic’s flu page.