Flu (Influenza)

Influenza is a highly contagious viral infection that primarily affects the respiratory system—the nose, throat, and lungs. Unlike the common cold, flu symptoms typically come on suddenly and are more severe. The flu is caused by influenza viruses that are constantly changing, which is why people can get the flu more than once and why flu vaccines are updated each year.

There are four types of influenza viruses: A, B, C, and D. Influenza A and B viruses cause seasonal epidemics of disease in people almost every winter in the United States. Influenza A viruses are the only influenza viruses known to cause flu pandemics. Type C infections generally cause mild respiratory illness and are not thought to cause epidemics. Type D viruses primarily affect cattle and are not known to infect people.

The flu spreads mainly by tiny droplets made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose, or eyes. People with flu are most contagious in the first 3-4 days after their illness begins, though some can spread the virus from 1 day before symptoms develop and up to 5-7 days after becoming sick.

Types of Flu Viruses

Influenza A: The most common type, responsible for seasonal flu epidemics. Subtypes are classified by two proteins on the surface: hemagglutinin (H) and neuraminidase (N). Common subtypes include H1N1 and H3N2. Influenza A viruses can infect people, birds, pigs, and other animals.

Influenza B: Also causes seasonal epidemics but generally causes milder illness than type A. Type B viruses are classified into two lineages: Victoria and Yamagata. They primarily infect humans and seals.

How Flu Spreads

• Airborne droplets: When infected people cough, sneeze, or talk
• Direct contact: Touching contaminated surfaces then touching face
• Close contact: Being within 6 feet of an infected person
• Before symptoms appear: People can spread flu 1 day before symptoms

Why Flu Changes

Influenza viruses are constantly changing through two mechanisms:

• Antigenic drift: Small changes in genes that happen continually as the virus replicates
• Antigenic shift: Abrupt, major changes resulting in new strains to which people have little immunity

People at Higher Risk for Complications

• Children younger than 5 years, especially those younger than 2 years
• Adults 65 years and older
• Pregnant women and women up to 2 weeks after pregnancy
• American Indians and Alaska Natives
• People with chronic medical conditions:
  • Asthma
  • Heart disease
  • Diabetes
  • Chronic kidney disease
  • Liver disorders
  • Neurologic conditions
  • Blood disorders
  • Metabolic disorders
• People with weakened immune systems due to disease or medication
• People with extreme obesity (BMI of 40 or higher)
• Residents of nursing homes and long-term care facilities

Environmental Risk Factors

• Living or working in crowded conditions
• Healthcare workers with patient exposure
• Teachers and childcare workers
• International travelers during flu season
• People attending large gatherings

Clinical Diagnosis

Healthcare providers often diagnose flu based on symptoms and physical examination, especially during flu season when the virus is widespread. Key indicators include sudden onset of fever, cough, and body aches during flu season.

Diagnostic Tests

Rapid Influenza Diagnostic Tests (RIDTs)

• Results in 10-15 minutes
• Lower sensitivity (50-70% accurate)
• Can miss some flu cases
• Most commonly used in clinics

Rapid Molecular Assays

• Results in 15-20 minutes
• More accurate than RIDTs
• Can detect flu virus genetic material
• Becoming more widely available

RT-PCR (Reverse Transcription Polymerase Chain Reaction)

• Most accurate flu test
• Results in 1-8 hours
• Can identify flu type and subtype
• Often used in hospitals

Viral Culture

• Takes 3-10 days for results
• Used for detailed virus analysis
• Helps track virus changes
• Not used for immediate diagnosis

Antiviral Medications

Antiviral drugs can lessen symptoms, shorten illness duration by 1-2 days, and prevent serious complications. They work best when started within 48 hours of symptom onset but can still provide benefit if started later, especially in high-risk patients.

FDA-Approved Flu Antivirals

• Oseltamivir (Tamiflu): Oral medication for ages 2 weeks and older
• Zanamivir (Relenza): Inhaled medication for ages 7 years and older
• Peramivir (Rapivab): IV medication for ages 2 years and older
• Baloxavir marboxil (Xofluza): Single-dose oral medication for ages 12 years and older

Supportive Care

• Rest: Get plenty of sleep to help your immune system
• Hydration: Drink water, juice, warm soups to prevent dehydration
• Fever reducers: Acetaminophen or ibuprofen for fever and aches
• Cough medicine: For severe cough that interferes with sleep
• Humidifier: Add moisture to ease congestion
• Throat lozenges: For sore throat relief

What NOT to Do

• Don't give aspirin to children or teenagers (risk of Reye's syndrome)
• Don't use antibiotics (they don't work against viruses)
• Don't return to work/school until fever-free for 24 hours without medication
• Avoid alcohol and smoking

Common Complications

• Bacterial pneumonia: Most common serious complication
• Ear infections: Especially common in children
• Sinus infections: Can develop after flu
• Bronchitis: Inflammation of bronchial tubes
• Dehydration: From fever and reduced fluid intake

Serious Complications

• Respiratory failure: May require mechanical ventilation
• Myocarditis: Inflammation of heart muscle
• Encephalitis: Brain inflammation
• Multi-organ failure: In severe cases
• Sepsis: Life-threatening response to infection
• Worsening of chronic conditions: Like asthma, diabetes, heart disease

Post-Flu Complications

• Secondary bacterial infections
• Post-viral fatigue syndrome
• Guillain-Barré syndrome (rare)
• Reye's syndrome (in children given aspirin)

Annual Flu Vaccination

The best way to prevent flu is to get vaccinated each year. The flu vaccine is updated annually to protect against the viruses most likely to cause illness in the upcoming flu season.

Types of Flu Vaccines

• Standard-dose shots: For people 6 months through 64 years
• High-dose shots: For people 65 years and older
• Nasal spray vaccine: For healthy people 2-49 years old
• Cell-based vaccines: Grown in cell culture instead of eggs
• Recombinant vaccines: Egg-free option for people with allergies

Who Should Get Vaccinated

• Everyone 6 months and older should get annual flu vaccine
• Especially important for high-risk groups
• Healthcare workers and caregivers
• Pregnant women (vaccine is safe during pregnancy)

Daily Preventive Actions

• Hand hygiene: Wash hands frequently with soap and water
• Avoid touching face: Especially eyes, nose, and mouth
• Cover coughs and sneezes: Use tissue or elbow
• Stay home when sick: Avoid spreading illness
• Clean surfaces: Disinfect frequently touched objects
• Avoid close contact: With sick people when possible
• Maintain healthy habits: Good nutrition, exercise, adequate sleep

During Flu Season

• Consider wearing masks in crowded indoor spaces
• Improve ventilation in indoor spaces
• Avoid large gatherings if flu is widespread
• Keep immune system strong with healthy lifestyle
• Stock up on supplies in case you get sick

Recovery Timeline

• Days 1-3: Sudden onset, worst symptoms
• Days 4-5: Fever typically breaks, some improvement
• Days 6-7: Significant improvement, still contagious
• Week 2: Most people fully recovered
• Weeks 2-3: Some may have lingering fatigue or cough

Home Care Tips

• Create a "sick room" if possible to isolate from family
• Use separate bathroom if available
• Have one family member be primary caregiver
• Keep tissues, hand sanitizer, and trash can nearby
• Monitor temperature regularly
• Keep a symptom diary
• Stay in contact with healthcare provider

When to Return to Normal Activities

• Wait until fever-free for 24 hours without fever-reducing medication
• Symptoms should be improving
• Still wash hands frequently after returning
• Wear mask if still coughing
• Avoid vulnerable people for several more days