Overview

Influenza (flu) is a highly contagious respiratory illness caused by influenza viruses. It primarily affects the nose, throat, and lungs. Unlike the common cold, flu symptoms often appear suddenly and can be severe.

Each year, seasonal influenza causes millions of illnesses in the United States, leading to hundreds of thousands of hospitalizations and thousands of deaths. The flu is most dangerous for older adults (especially those 65+), young children, pregnant individuals, and people with chronic health conditions such as asthma, heart disease, or diabetes.

Prevalence and Seasonality

Flu activity typically begins in the fall, peaks between December and February, and can continue into spring. Annual case numbers vary widely, with estimates ranging from 9 million to 41 million infections, 140,000 to 710,000 hospitalizations, and 12,000 to 52,000 deaths depending on the severity of the season.

Symptoms

Flu symptoms often develop abruptly and affect the entire body, not just the respiratory system. Common symptoms include:

• Fever or chills: A high temperature often comes on quickly.
• Cough and sore throat: A dry, hacking cough is typical, along with a painful or scratchy throat.
• Runny or stuffy nose: Nasal congestion can make it hard to breathe.
• Body aches and headaches: Muscles and joints may feel tender and sore.
• Fatigue: Extreme tiredness can make even simple tasks feel exhausting.
• Vomiting and diarrhea: These are more common in children but can occur in adults as well.: These are more common in children but can occur in adults as well.

Seek medical care if any of the following occur:

• Difficulty breathing or shortness of breath
• Persistent chest pain or pressure
• Sudden dizziness, confusion, or fainting
• Severe weakness or inability to stay awake
• Fever that worsens or does not improve after several days

Flu symptoms can resemble other respiratory illnesses, but their rapid onset and intensity are distinguishing features.

Complications

While many recover fully within a week, influenza can lead to serious complications, especially in high-risk groups. These include:

• Pneumonia (viral or bacterial)
• Sinus and ear (more common in children) infections
• Worsening of chronic conditions (e.g., asthma, heart disease, diabetes)
• Inflammation of the heart (myocarditis), brain (encephalitis), or muscles (myositis)

During severe seasons, a significant portion of hospitalized flu patients have underlying health conditions. In the 2024–25 season, 89.1% of hospitalized patients had at least one chronic illness, 16.8% required intensive care, and 3% died in hospital.

Causes

The flu is caused by influenza viruses, primarily types A and B, which infect the respiratory tract—nose, throat, and lungs. These viruses are constantly evolving through genetic changes known as antigenic drift, which is why new strains emerge each year and annual vaccination is recommended.

Transmission

Influenza spreads easily from person to person, mainly through respiratory droplets produced when an infected person coughs, sneezes, or talks. These droplets can travel up to six feet and land in the mouths or noses of nearby individuals.

Less commonly, the virus can spread by touching contaminated surfaces (e.g., doorknobs, shopping carts) and then touching your eyes, nose, or mouth.

Flu transmission is especially rapid in crowded indoor environments such as schools, offices, and long-term care facilities, particularly during colder months when people spend more time indoors.

Because the virus changes frequently, prior immunity may not protect against new strains, making reinfection possible even within the same season.

Risk Factors

Anyone can get the flu, but certain groups face a higher chance of severe illness, hospitalization, or complications.

Higher risk groups include:

• Young children (under age 5) and adults over age 65
• Pregnant women
• People with chronic health conditions (e.g. asthma, diabetes, or heart disease)
• People with weakened immune systems
• Residents of nursing homes or other long-term care facilities

Risk factors for exposure to the virus itself include being in crowded areas, living with or caring for someone who is sick, and being exposed during peak flu season.

Diagnosis

If you think you have the flu, a healthcare provider can often diagnose you based on your symptoms alone, especially during the flu season. However, they may also use a test to confirm the diagnosis.

Common tests include:

• Rapid influenza diagnostic tests (RIDTs): These quick tests use a nasal swab to check for the flu virus. While they can provide a result in about 10–15 minutes, they may not always be accurate, and a negative result doesn't rule out the flu.
• Rapid molecular assays: These tests are more reliable than RIDTs. They detect the genetic material of the virus and can provide results in under 20 minutes.

Treatment

Influenza treatment focuses on reducing symptom severity and duration while preventing complications. Options include prescription antivirals and over-the-counter (OTC) relief.

Prescription Antivirals

Antiviral medications can shorten flu duration by 1–2 days and may prevent infection if taken after exposure. They work best when started within 48 hours of symptom onset:

• Oseltamivir (Tamiflu): Oral pill or liquid, approved for treatment in individuals 14 days and older, also used for prevention.
• Zanamivir (Relenza): Inhaled powder via Diskhaler, approved for treatment in patients 7 years and older, and for prevention. Not recommended for those with respiratory conditions like asthma or COPD.
• Peramivir (Rapivab): Single-dose IV, approved for treatment in patients 6 months and older, suitable for those with kidney issues.
• Baloxavir marboxil (Xofluza): Single-dose oral pill, approved for treatment in children aged 5 to under 12 without chronic conditions, and individuals 12 and older.

Some antivirals, including Tamiflu, Relenza, and Xofluza, may also be prescribed for preventive use after exposure to the flu virus.

Over-the-Counter Relief

OTC medications do not treat the virus itself but can help relieve symptom.

Children typically continue low-dose aspirin for 6 to 8 weeks, or longer if heart issues are detected.

Prevention

Currently, there is no known way to prevent Kawasaki disease, as its exact cause remains unclear. Unlike many childhood illnesses, there is no vaccine or specific preventive measure available.

What Parents Can Do

• Early recognition is key. If a child has a fever lasting more than 5 days, especially with signs like red eyes, rash, or swollen hands and feet, they should be evaluated by a doctor immediately.
• Prompt treatment significantly reduces the risk of heart complications and improves outcomes.

While Kawasaki disease itself cannot be prevented, early action and close follow-up are the best ways to protect a child’s heart and overall health.