flu (influenza)

Overview
Influenza, commonly known as the flu, is a highly contagious viral infection that primarily affects the respiratory system, such as the nose, throat, and lungs. It spreads easily through droplets when infected people cough, sneeze, or talk. Unlike the common cold, the flu usually comes on suddenly and can cause severe symptoms, including high fever, fatigue, and body aches. Each year, seasonal flu viruses circulate globally, causing widespread illness and sometimes serious complications.

Why It’s Important
Though often mistaken as a mild illness, the flu can be dangerous, especially for young children, older adults, and people with underlying health conditions. It leads to hundreds of thousands of hospitalizations and tens of thousands of deaths in the U.S. each year. The flu also puts a major burden on healthcare systems and can contribute to secondary infections like pneumonia. Annual vaccination and early treatment are key to reducing severity and spread.

Quick Facts

  • Affects people of all ages, but severe cases are more common in the very young, elderly, and immunocompromised

  • 5–20% of the U.S. population gets the flu each year (CDC)

  • Spread through respiratory droplets and surface contact

  • Typically peaks in fall and winter months (flu season)

  • Caused by influenza viruses A, B, C, and D (A and B are most common in humans)

What Happens in the Body
Once inhaled or transferred via contact, the influenza virus infects the cells lining the respiratory tract, triggering the immune system to respond. This response causes inflammation, leading to fever, fatigue, and muscle aches. In some cases, the infection can spread to the lungs, leading to bronchitis or pneumonia, especially in vulnerable individuals. The body eventually clears the virus, but lingering symptoms like fatigue may persist for weeks.

Common Signs and Symptoms

  • Sudden onset of fever and chills

  • Dry cough

  • Sore throat

  • Runny or stuffy nose

  • Muscle or body aches

  • Fatigue or weakness

  • Headache

  • Loss of appetite

  • Vomiting and diarrhea (more common in children)

Diagnosis

  • Often diagnosed based on symptoms and time of year

  • Rapid influenza diagnostic tests (RIDTs) can confirm flu within minutes

  • PCR tests offer higher accuracy and are used in hospitals
    See a doctor if:

  • You are in a high-risk group and experience symptoms

  • You have difficulty breathing, chest pain, dizziness, or symptoms that worsen after improving

  • A child has bluish skin, trouble breathing, or irritability

Treatment and Management

  • Antiviral medications like oseltamivir (Tamiflu) or baloxavir (Xofluza) work best if started within 48 hours of symptom onset

  • Fluids, rest, and over-the-counter medications (acetaminophen or ibuprofen) help manage symptoms

  • Annual flu vaccine is the most effective prevention tool

  • Hand hygiene, masking, and avoiding sick contacts reduce spread

  • In severe cases, hospitalization may be needed, especially for pneumonia or dehydration

Risks and Complications

  • Pneumonia

  • Sinus and ear infections

  • Myocarditis (heart inflammation)

  • Exacerbation of chronic illnesses (e.g., asthma, diabetes, heart disease)

  • Hospitalization and death, particularly in high-risk groups

Who Is at Risk

  • Adults 65 and older

  • Children under 5, especially under age 2

  • Pregnant women

  • People with asthma, heart disease, diabetes, obesity, or weakened immune systems

  • Healthcare workers and caregivers

Related Conditions

  • Pneumonia

  • Bronchitis

  • Secondary bacterial infections

  • COVID-19 (some overlapping symptoms)

  • Reye’s syndrome (in children who take aspirin)

When to See a Doctor

  • If symptoms are severe or worsen after initially improving

  • If you are in a high-risk group and develop flu symptoms

  • If you have difficulty breathing, confusion, persistent fever, or dehydration

Last Updated: June 28, 2025