EVIDENCE-BASEDAWAITING CLINICIAN REVIEW

Influenza (flu)

Influenza (flu) is a contagious respiratory illness caused by influenza viruses. It often starts abruptly with fever or chills, muscle aches, headache, fatigue and cough, but illness ranges from mild to severe and can cause pneumonia, worsen chronic disease or lead to hospitalization in people at higher risk.

At a glance

Quick facts

Condition snapshot
Cause
Influenza viruses
Onset
Often abrupt
Common symptoms
Fever/chills · aches · cough · fatigue
Testing
Molecular tests when results affect care
Treatment
Antivirals for selected / high-risk patients
Prevention
Seasonal vaccination

Names & terminology

How this entity may be named or searched

Influenza
A respiratory infection caused by influenza viruses, distinct from the many other viruses that cause “flu-like” illness.
Seasonal influenza
The recurring annual circulation of influenza A and B viruses.
Influenza-like illness
A symptom-based surveillance term; it is not proof of influenza without testing.
Antiviral treatment
Prescription medicine active against influenza; antibiotics do not treat the influenza virus itself.

Structured entity profile

Clinical profile

Condition data
TransmissionRespiratory droplets/aerosols + contactSpread is greatest around the symptomatic period but can begin before symptoms.
Typical onsetSudden systemic + respiratory symptomsFever is common but not universal.
CourseOften 3–7 days of acute symptomsCough and malaise may persist beyond two weeks in some people.
ComplicationsPneumonia and chronic-disease exacerbationRisk is higher at age extremes, pregnancy and with certain chronic diseases.
TreatmentNeuraminidase or cap-dependent antiviralsBenefit is greatest when started early; severe/high-risk patients can benefit even when started later.
PreventionAnnual seasonal vaccinationVaccine composition is updated as circulating viruses evolve.

Overview

Influenza viruses infect the nose, throat and lungs. Unlike many common colds, uncomplicated influenza often begins abruptly with prominent systemic symptoms such as fever, chills, myalgia, headache and marked fatigue, followed by or accompanied by cough, sore throat and rhinitis. Not everyone has a fever, especially older or immunocompromised people. [1] [2]

Most otherwise healthy people recover without complication, but influenza can cause primary viral pneumonia, secondary bacterial pneumonia, dehydration, myocarditis, encephalopathy or worsening of asthma, COPD, heart failure, diabetes and other chronic conditions. Young children, older adults, pregnant people and people with certain chronic diseases or immune compromise have higher complication risk. [1] [3]

Symptoms and warning signs

Typical symptoms are fever or feeling feverish, chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headache and fatigue. Vomiting or diarrhea occur more often in children than adults. In older adults, atypical presentations such as loss of appetite or acute confusion can occur. Difficulty breathing, chest pain, cyanosis, dehydration, seizures, severe weakness or improvement followed by sudden worsening are warning signs for complications. [1] [3]

SYMPTOM MATRIX

Influenza is a systemic respiratory illness — but presentations vary by age and risk

The absence of fever does not exclude influenza.

Acute respiratory infection
Typical symptoms
  • Abrupt fever or chills
  • Cough
  • Muscle or body aches
  • Headache
  • Marked fatigue
  • Sore throat / rhinitis
Age / risk variations
  • No fever
  • Vomiting or diarrhea, especially in children
  • Loss of appetite
  • Confusion in frail older adults
  • Worsening of asthma or other chronic disease
Warning signs
  • Difficulty breathing or low oxygen
  • Chest pain or pressure
  • Severe dehydration
  • Seizure or persistent confusion
  • Symptoms improve then return much worse

Severe or high-risk influenza may need urgent assessment and antiviral treatment.

Diagnosis

During periods of known influenza circulation, clinicians can diagnose uncomplicated influenza from the clinical pattern, especially when testing would not change management. Testing becomes more useful in hospitalized patients, severe disease, outbreak settings, immunocompromised people or when the result will influence antiviral, antibiotic or infection-control decisions. [2] [3]

Rapid molecular assays and laboratory RT-PCR are more sensitive than older rapid antigen tests. A negative low-sensitivity antigen test does not reliably exclude influenza when clinical suspicion is high. Testing should not delay empiric antiviral treatment in hospitalized or high-risk patients when influenza is suspected. [2] [3]

Diagnostic pathway

Use clinical context first; test when the answer changes management

High-risk and hospitalized patients should not wait for a test result before indicated antiviral therapy.

Respiratory diagnosis
1Assess symptoms and local circulationAbrupt systemic symptoms during influenza season increase pretest probability.
2Identify high-risk featuresAge, pregnancy, chronic disease, immune status and severity change treatment thresholds.
3Test when usefulPrefer molecular testing in hospitalized or high-consequence situations.
4Treat promptly when indicatedStart antiviral therapy without waiting for confirmation in severe/high-risk suspected influenza.
Preferred sensitive testMolecular / RT-PCRespecially hospitalized
Best antiviral timingwithin 1–2 daysbut later still useful in severe/high-risk disease

The decision to test is different from the decision to treat; treatment should not be delayed in severe or high-risk suspected influenza.

Who is at higher risk of serious flu complications?

Risk groups include young children, adults 65 and older, pregnant people and those within the postpartum period, residents of long-term-care facilities, and people with chronic lung, heart, neurologic, kidney, liver, metabolic or immune conditions. Obesity at certain severity thresholds is also a recognized risk factor. High-risk status lowers the threshold for early antiviral treatment and clinical review. [3]

Complications

Influenza can directly cause viral pneumonia and can set the stage for bacterial pneumonia. It can exacerbate asthma, COPD, heart disease and diabetes and, less commonly, cause myocarditis, encephalitis, myositis or multi-organ failure. A second deterioration after initial improvement can signal pneumonia or another complication. [1] [3]

Treatment and management

Most low-risk people with mild influenza recover with rest, fluids and symptom care. Prescription influenza antivirals can shorten illness and reduce complications in selected patients. CDC recommends prompt antiviral treatment for people who are hospitalized, have severe or progressive illness, or are at higher risk of complications; treatment works best when started within one to two days but can still benefit severe or high-risk patients when started later. [2] [3]

Antibiotics do not treat influenza and should be reserved for suspected or confirmed bacterial complications. Antiviral choice depends on age, pregnancy, kidney function, route of administration, circulating resistance and contraindications. People with severe disease should not rely on over-the-counter symptom medicines alone. [2] [3]

Treatment & care hub

Treat risk and severity, not just the calendar day

Early antiviral therapy matters most in high-risk or severe disease.

Seasonal flu care
AntiviralsHigh risk / severe

Start promptly when indicated

Benefit is greatest early, but treatment can still help hospitalized or high-risk patients after 48 hours.

Supportive careMost mild illness

Hydration, rest and symptom relief

Low-risk uncomplicated cases often recover without prescription treatment.

Bacterial complicationsIf suspected

Use antibiotics selectively

Antibiotics treat bacterial pneumonia or other bacterial complications—not influenza virus.

Chronic diseaseMonitor

Watch for exacerbations

Asthma, COPD, cardiac and metabolic disease can destabilize during influenza.

Assess risk→Treat early→Watch complications→Prevent next season

Antiviral selection and dosing should follow current local guidance because age, pregnancy, kidney function and resistance matter.

Vaccination and everyday prevention

Seasonal influenza vaccination is the main preventive intervention. CDC recommends routine annual influenza vaccination for everyone aged 6 months and older in the United States, with product and timing recommendations updated each season. Hand hygiene, improved ventilation, staying home while acutely ill and masking in selected high-risk settings can further reduce respiratory-virus spread. [4]

Recovery and return to normal activity

Acute symptoms often resolve over three to seven days, but cough and fatigue can persist for more than two weeks, especially in older adults and people with chronic lung disease. A prolonged cough alone does not necessarily mean ongoing contagious influenza, but persistent fever, worsening breathlessness or a new decline should prompt reassessment. [1]

Clinical course

Uncomplicated influenza usually improves within about 3–7 days

Cough and malaise can last longer, particularly in older adults and people with chronic lung disease.

CDC — Clinical Signs and Symptoms of Influenza, Feb 2026 ↗
Acute uncomplicated symptoms~3–7 daysfor most people
Cough / malaisemay persist >2 weeks
Antivirals work bestwithin 1–2 days
Note: Severe disease and high-risk patients can have a very different course and need earlier clinical review.

Common questions

Frequently asked questions

Reference content

Sources

  1. CDC — Clinical Signs and Symptoms of Influenza — Feb 2026
  2. CDC — Treatment of Flu
  3. CDC — People at Increased Risk for Flu Complications
  4. CDC — Preventing Seasonal Flu

This article is based on trusted medical sources and is awaiting clinician review. It is not a substitute for personalized medical advice.