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Influenza (the flu) is a contagious infection of the airways caused by influenza viruses. Unlike a common cold it often comes on suddenly with fever, aches, headache and exhaustion, as well as cough and sore throat. Most healthy people recover in a week or so, but the flu can be serious for older adults, young children, pregnant women and people with heart or lung conditions. A yearly vaccine is the best protection. This page explains influenza, its complications, and the physiotherapy role in recovery.
Definition
Influenza (the flu) is an acute respiratory infection caused by influenza A and B viruses, which circulate seasonally — mainly in the cooler months in Australia — and change from year to year. It is genuinely distinct from the common cold: the onset is abrupt, the systemic illness is disproportionate to the respiratory symptoms, and it causes deaths every season. Treating it as a bad cold is the most common clinical misjudgement associated with it.
Pathophysiology
How the virus damages the airway
Influenza binds sialic acid receptors on respiratory epithelium and replicates in the upper and lower airways, destroying ciliated cells and stripping the mucociliary escalator. That epithelial denudation explains both the persistent post-viral cough and the marked susceptibility to secondary bacterial infection over the following two weeks.
Why the systemic illness is so severe
Most of the fever, aches, headache and prostration is the host inflammatory response rather than direct viral injury — a surge of interferons and pro-inflammatory cytokines. This is why the illness feels systemic out of proportion to the cough, and why symptoms peak and then break relatively abruptly.
Why chronic disease destabilises
Systemic inflammation increases metabolic demand, provokes bronchial hyper-responsiveness, raises heart rate and myocardial oxygen demand, and promotes a prothrombotic and plaque-destabilising state. In someone with limited cardiac or respiratory reserve, the decompensation of the underlying condition — not the viral pneumonia — is usually what causes harm.
Antigenic drift and shift
Small mutations in surface antigens (drift) mean immunity from previous years is incomplete, requiring annual reformulation of the vaccine. Major reassortment (shift) in influenza A can produce a novel virus against which the population has no immunity, which is the mechanism of pandemics.
Co-morbidities
Influenza is more dangerous for adults over 65, children under five, pregnant women and those up to two weeks postpartum, Aboriginal and Torres Strait Islander people, people with obesity, and anyone with chronic heart, lung, kidney, liver, neurological or metabolic disease or reduced immunity. Asthma and COPD exacerbations, decompensated heart failure and diabetic decompensation are the common patterns. These groups are prioritised for vaccination and for early antiviral treatment regardless of how well they look at presentation.
Prevalence
Seasonal influenza causes large annual epidemics, with hundreds of thousands of notified cases in Australia in a significant season and an estimated three to five million cases of severe illness worldwide each year. Notified cases substantially understate the true burden, since most people are never tested. Activity, timing and dominant strain vary considerably season to season, and the northern hemisphere season is an imperfect predictor of ours.
Causes and risk factors
- Infection with influenza A or B, spread by respiratory droplets, aerosols and contaminated surfaces.
- Contagious before symptoms — from about a day before onset until five to seven days after, and longer in children and immunocompromised people.
- Not being vaccinated, or vaccinated more than a season ago.
- Age at either extreme, pregnancy, and chronic disease or immunosuppression.
- Crowding and indoor contact — households, schools, workplaces and residential aged care.
- Attending work or school while unwell, which is the principal driver of workplace and school transmission.
Symptoms
Typical features
- Abrupt onset of fever and chills — patients can often name the hour it started.
- Prominent muscle and joint aches, headache and profound fatigue.
- Dry cough, sore throat and runny nose.
- Gastrointestinal symptoms, more common in children.
- Symptoms peak over two to three days and settle within about a week; cough and fatigue commonly persist two to three weeks longer.
Influenza versus a cold
The distinction is clinically useful: colds begin gradually with prominent nasal symptoms and little systemic upset, while influenza begins abruptly with fever, myalgia and prostration and comparatively modest early nasal symptoms. Neither is reliable enough to base treatment on alone, but the pattern should prompt testing in an at-risk patient.
Presentation in older and frail patients
Fever may be absent. Influenza in a frail older adult frequently presents as delirium, a fall, functional decline, or decompensation of heart failure or COPD. Missing it in this group is common and consequential.
Warning signs
Diagnosis
Why diagnosis matters
Recognising influenza allows early antiviral treatment in those at risk, prompts isolation to protect vulnerable contacts, avoids unnecessary antibiotics, and correctly attributes an exacerbation. In hospital and residential aged care it also drives outbreak control.1 Laboratory-confirmed influenza is a notifiable disease in Queensland and across Australia: confirmed cases are reported to the public health unit, which is how outbreaks in schools and aged care are detected.1
How is it diagnosed?
Diagnosis is usually clinical during a recognised season. Where it changes management — in hospital, in high-risk patients, or during an outbreak — PCR on a nose or throat swab is the reference test. Rapid antigen tests are quicker but less sensitive, so a negative result during peak season does not exclude the diagnosis in a typical case.
Radiology
Chest X-ray is used when pneumonia is suspected. Primary influenza pneumonia produces diffuse bilateral infiltrates; a focal lobar pattern, particularly appearing after initial improvement, suggests secondary bacterial pneumonia and different treatment.
Investigations for related conditions
If breathing worsens or fever persists or recurs, investigation looks for secondary bacterial pneumonia, myocarditis, myositis with rhabdomyolysis, or decompensation of an underlying condition — with blood gases, inflammatory markers, creatine kinase, troponin, natriuretic peptides and imaging as indicated.
Management
Management and goals
Goals are to relieve symptoms, shorten illness and prevent complications in those at risk, treat any destabilised underlying condition, limit transmission, and identify deterioration early. For most healthy adults this amounts to rest, fluids and staying away from other people.
Antiviral treatment
Neuraminidase inhibitors — oseltamivir and zanamivir — shorten symptom duration and reduce lower-respiratory complications requiring antibiotics and hospital admission, with the effect greatest when started within 48 hours of onset.2 Baloxavir, a single-dose cap-dependent endonuclease inhibitor, achieved comparable symptom reduction to oseltamivir with faster viral clearance in uncomplicated influenza.3 Treatment is recommended for hospitalised, severe or high-risk patients, and is still worth giving beyond 48 hours in severe or progressive illness.
Corticosteroids — a caution
Adjunctive corticosteroids are not recommended in influenza. A Cochrane review found their use associated with increased mortality and higher rates of hospital-acquired infection, albeit on low-quality observational evidence.4 They are given only where indicated for another reason, such as an asthma or COPD exacerbation.
Severe disease and critical care
Severe influenza can progress to acute respiratory distress syndrome requiring lung-protective ventilation. Prone positioning significantly reduced mortality in severe ARDS5 and is standard in that setting. Extracorporeal membrane oxygenation was used extensively for refractory influenza-related ARDS during the 2009 H1N1 pandemic, with survival to discharge in a substantial proportion of an extremely sick Australian and New Zealand cohort.6
Vaccination
Annual vaccination is the cornerstone of prevention. Efficacy varies by season and by the match between vaccine and circulating strain, and is moderate rather than complete — a systematic review found pooled efficacy of around 59% in healthy adults where the strain was well matched.7 In older adults, high-dose vaccine was significantly more effective than standard dose at preventing laboratory-confirmed influenza.8 Australian recommendations, including which formulation applies at which age, are set out in the Immunisation Handbook and updated annually.9
Identifying deterioration
A second fever after initial improvement, increasing breathlessness, pleuritic chest pain, purulent sputum, confusion or an oxygen saturation below baseline all suggest complication rather than ordinary course. Deterioration around days five to seven, after apparent improvement, is the classic pattern of secondary bacterial pneumonia.
Medications
Medications for influenza
Antivirals as above; paracetamol or ibuprofen for fever and aches. Antibiotics have no role in uncomplicated influenza and are reserved for secondary bacterial infection, where Staphylococcus aureus and Streptococcus pneumoniae predominate. People with asthma or COPD continue their usual inhalers and start their written action plan.
Correct use of medications
Antivirals work best within 48 hours, which means the practical barrier is access rather than efficacy — high-risk patients should know in advance to seek care on day one rather than day four. Oseltamivir requires dose adjustment in renal impairment and commonly causes nausea, which is reduced by taking it with food. Aspirin is avoided in children and adolescents because of the risk of Reye syndrome.
Multi-system manifestations
Cardiovascular
Influenza substantially increases the short-term risk of acute myocardial infarction and stroke in the days following infection, and can cause myocarditis and pericarditis. Decompensated heart failure is one of the commonest reasons an older person with influenza is admitted.
Neurological
Febrile seizures in young children; encephalitis and encephalopathy, uncommon but serious; and Guillain–Barré syndrome as a rare post-infectious complication. Delirium in frail older adults is common and often the presenting feature.
Muscle
Myositis, particularly in children with influenza B, causing calf pain and refusal to walk; and rarely rhabdomyolysis with acute kidney injury, which is why creatine kinase is checked where muscle pain is disproportionate.
Renal and metabolic
Acute kidney injury from dehydration, sepsis or rhabdomyolysis, and destabilisation of diabetes with hyperglycaemia or ketoacidosis driven by systemic inflammation and reduced oral intake.
Secondary bacterial infection
The classic and most important complication: bacterial pneumonia developing as the initial illness appears to settle, with a second fever, focal signs and purulent sputum. It carries appreciable mortality and is the reason a returning fever is never dismissed.
Pregnancy
Influenza in pregnancy carries higher risk of severe illness, hospitalisation and intensive care, and is associated with preterm birth and low birthweight. Vaccination in pregnancy protects both the mother and the infant in the first months of life.
Living with influenza
Recovery at home
Rest, generous fluids, simple analgesia and staying away from others while infectious. Most healthy adults are through the acute illness in about a week, but fatigue and cough persisting two to three weeks are normal and do not indicate treatment failure.
Nutrition and hydration
Appetite falls sharply, and dehydration from fever, sweating and reduced intake is a common reason for presentation, particularly in older adults and children. Small frequent fluids matter more than food in the acute phase.
Sleep and pacing
Sleep is disrupted by fever, cough and aches. Returning to full activity before fatigue has resolved commonly produces a relapse in symptoms, and a staged return over one to two weeks is more effective than a hard resumption.
Protecting others
Staying home while febrile and for at least 24 hours after fever resolves, hand hygiene, covering coughs, and avoiding contact with infants, pregnant women, older relatives and immunosuppressed people. In residential aged care and hospital, this is formal outbreak management.
Annual vaccination as part of the action plan
For anyone with chronic lung or heart disease, annual influenza vaccination belongs in the written action plan alongside their inhalers — as does knowing that an influenza illness is a trigger to start that plan and seek review early rather than wait.
Prognosis
Healthy people recover fully, though fatigue and cough often outlast the fever by weeks. Influenza is serious or fatal in high-risk groups, largely through secondary bacterial pneumonia, cardiovascular events and decompensation of underlying disease rather than the viral illness alone. Vaccination and early antivirals substantially reduce severe outcomes. In frail older adults, an episode frequently marks a lasting step down in function — a strong argument for active rehabilitation after the acute illness rather than simple reassurance.
Role of the physiotherapist
Physiotherapy is not needed for uncomplicated influenza, and routine chest treatment in an acute febrile viral illness is neither indicated nor tolerated. The role lies in complications and in recovery.
Where influenza triggers an exacerbation of a chronic lung condition, treatment follows that condition: airway clearance for retained secretions in bronchiectasis or COPD, breathing retraining and positioning for breathlessness, and support with non-invasive ventilation where indicated. Where influenza causes pneumonia, the evidence for conventional chest physiotherapy in uncomplicated adult pneumonia is limited — a Cochrane review found no reduction in mortality or cure rate10 — so effort is directed at positioning, mobilisation, lung expansion and reconditioning rather than routine percussion.
In critical illness, physiotherapy contributes to positioning including prone care, weaning support, and early mobilisation to limit ICU-acquired weakness. Afterwards, reconditioning is the highest-value contribution: a graded return to activity and cardiorespiratory rehabilitation to rebuild the fitness, muscle mass and confidence lost to a severe illness or hospital stay, with falls-risk review in older patients. Physiotherapists are also well placed to check influenza vaccination status at review — a brief intervention with a disproportionate effect in exactly the population they see most.
Part 1 · References
- Uyeki TM, Bernstein HH, Bradley JS, et al. Clinical practice guidelines by the Infectious Diseases Society of America: 2018 update on diagnosis, treatment, chemoprophylaxis, and institutional outbreak management of seasonal influenza. Clin Infect Dis 2019;68(6):e1–e47.
- Dobson J, Whitley RJ, Pocock S, Monto AS. Oseltamivir treatment for influenza in adults: a meta-analysis of randomised controlled trials. Lancet 2015;385(9979):1729–1737.
- Hayden FG, Sugaya N, Hirotsu N, et al. Baloxavir marboxil for uncomplicated influenza in adults and adolescents. N Engl J Med 2018;379(10):913–923.
- Lansbury L, Rodrigo C, Leonardi-Bee J, Nguyen-Van-Tam J, Lim WS. Corticosteroids as adjunctive therapy in the treatment of influenza. Cochrane Database Syst Rev 2019;(2):CD010406.
- Guérin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome (PROSEVA). N Engl J Med 2013;368(23):2159–2168.
- Australia and New Zealand Extracorporeal Membrane Oxygenation (ANZ ECMO) Influenza Investigators. Extracorporeal membrane oxygenation for 2009 influenza A(H1N1) acute respiratory distress syndrome. JAMA 2009;302(17):1888–1895.
- Osterholm MT, Kelley NS, Sommer A, Belongia EA. Efficacy and effectiveness of influenza vaccines: a systematic review and meta-analysis. Lancet Infect Dis 2012;12(1):36–44.
- DiazGranados CA, Dunning AJ, Kimmel M, et al. Efficacy of high-dose versus standard-dose influenza vaccine in older adults. N Engl J Med 2014;371(7):635–645.
- Australian Technical Advisory Group on Immunisation. Australian Immunisation Handbook: influenza. Canberra: Department of Health and Aged Care; 2025.
- Yang M, Yan Y, Yin X, et al. Chest physiotherapy for pneumonia in adults. Cochrane Database Syst Rev 2013;(2):CD006338.
References are numbered in citation order (Vancouver/BMJ style) and were current at the time of writing. Guidelines are living documents — verify against the latest version before clinical use.
Supervised exercise, breathing technique and self-management education are the mainstay of cardiorespiratory physiotherapy for this condition.
Clinical evidence
Part 1 covers the same condition without the technical detail. What follows is the evidence base behind it, written for clinicians — the literature, the reasoning and the gaps.
For clinicians: this summary supports clinical reasoning and is not a protocol. Check current guidelines and local policy before applying it, and read it alongside the key references and guidelines directory.
Framing. Influenza is prevented rather than treated: annual vaccination, with enhanced formulations in older adults, is the intervention with the largest population effect, while antivirals shorten symptoms modestly and are most useful when started early in those at risk of complications.1,2,3 For physiotherapy the important evidence is negative — chest physiotherapy does not help uncomplicated influenza or pneumonia — and the important role is in the complicated patient: exacerbated chronic lung disease, secondary bacterial pneumonia, ARDS and post-illness deconditioning.4
Prevention and antiviral therapy
- Vaccine effectiveness is real but moderate and varies by season and match; pooled estimates in healthy adults are around 60% against laboratory-confirmed influenza in well-matched years.1
- High-dose and adjuvanted vaccines outperform standard-dose in adults over 65, with a randomised trial showing greater protection against laboratory-confirmed illness — the basis for age-specific formulations in the Australian programme.2,5
- Oseltamivir shortens symptom duration by around a day in adults and reduces lower respiratory tract complications requiring antibiotics, with nausea and vomiting the main harms; benefit depends on starting within 48 hours.3
- Baloxavir reduces symptom duration comparably to oseltamivir with faster viral clearance in a single dose, though treatment-emergent resistance is described.6
- Corticosteroids should not be used for influenza itself — observational and pooled data associate them with higher mortality and longer ICU stay.7
The complicated patient
- Chest physiotherapy does not improve outcomes in pneumonia in adults — no reduction in mortality or cure rate across trials of conventional techniques and osteopathic manipulation.4
- Influenza is a major trigger of asthma and COPD exacerbations and of decompensated heart failure, and these — not the virus — are what physiotherapy actually treats.8
- Secondary bacterial pneumonia (notably Staphylococcus aureus and Streptococcus pneumoniae) is the classic cause of the second deterioration after apparent improvement.8
- In severe influenza-associated ARDS, lung-protective ventilation, prone positioning and referral for extracorporeal support where indicated define management; the ANZ H1N1 experience established the referral pathway in this region.9,10
Physiotherapy implications
- Do not treat an uncomplicated influenzal chest. There is no clearance target, and treatment consumes PPE, staff and patient reserve.4
- Treat the exacerbated underlying disease on its own merits — bronchodilator optimisation and airway clearance in bronchiectasis or a productive COPD exacerbation, NIV in hypercapnic failure, positioning and mobilisation in the deteriorating patient.8
- Observe droplet and contact precautions, and treat nebulisation, high-flow oxygen, NIV, suctioning and cough-provoking techniques as higher-risk procedures requiring appropriate PPE and room selection.
- Mobilise early in hospitalised patients — the functional decline in an older person after a week of influenza is often the reason they do not return home.
- Advocate vaccination for patients with chronic cardiorespiratory disease and for staff; a physiotherapy appointment is a legitimate place to check vaccination status and prompt it.1,5
- Escalate a second fever spike after initial improvement, rising oxygen requirement, focal consolidation signs, confusion, or exhaustion with a falling respiratory rate.
- Expect a fatigue tail of weeks in older and multimorbid patients, and treat it with graded activity rather than investigating it as a new disease.
Clinical reasoning
- Ask what is actually wrong: viral illness alone (supportive care), bronchospasm (medical optimisation), secretion retention from underlying suppurative disease (clearance), consolidation (antibiotics and positioning), or fatigue and failure (escalation).
- A crackly chest in influenza is not automatically a clearance indication — it is often consolidation or bronchospasm.
- Deterioration after a period of improvement points to bacterial superinfection or a cardiac complication, including myocarditis.
- The frail older patient's dominant problem is usually functional decline rather than respiratory; prioritise mobility.
Evidence gaps
- No physiotherapy intervention has been tested specifically in influenza; the evidence is borrowed from pneumonia and exacerbation populations.4
- Optimal timing and dose of rehabilitation after severe influenza, including post-ICU, remain undefined.
- Aerosol-generating-procedure risk during respiratory physiotherapy is still estimated rather than measured.
- Long-term functional outcomes after severe influenza are far less well described than after COVID-19.
References for the clinical evidence summary
- Osterholm MT, Kelley NS, Sommer A, Belongia EA. Efficacy and effectiveness of influenza vaccines: a systematic review and meta-analysis. Lancet Infect Dis 2012;12(1):36–44.
- DiazGranados CA, Dunning AJ, Kimmel M, et al. Efficacy of high-dose versus standard-dose influenza vaccine in older adults. N Engl J Med 2014;371(7):635–645.
- Dobson J, Whitley RJ, Pocock S, Monto AS. Oseltamivir treatment for influenza in adults: a meta-analysis of randomised controlled trials. Lancet 2015;385(9979):1729–1737.
- Yang M, Yan Y, Yin X, et al. Chest physiotherapy for pneumonia in adults. Cochrane Database Syst Rev 2013;(2):CD006338.
- Australian Technical Advisory Group on Immunisation. Australian Immunisation Handbook: influenza. Canberra: Department of Health and Aged Care; 2025.
- Hayden FG, Sugaya N, Hirotsu N, et al. Baloxavir marboxil for uncomplicated influenza in adults and adolescents. N Engl J Med 2018;379(10):913–923.
- Lansbury L, Rodrigo C, Leonardi-Bee J, Nguyen-Van-Tam J, Lim WS. Corticosteroids as adjunctive therapy in the treatment of influenza. Cochrane Database Syst Rev 2019;(2):CD010406.
- Uyeki TM, Bernstein HH, Bradley JS, et al. Clinical practice guidelines by the Infectious Diseases Society of America: 2018 update on diagnosis, treatment, chemoprophylaxis, and institutional outbreak management of seasonal influenza. Clin Infect Dis 2019;68(6):e1–e47.
- Australia and New Zealand Extracorporeal Membrane Oxygenation (ANZ ECMO) Influenza Investigators. Extracorporeal membrane oxygenation for 2009 influenza A(H1N1) acute respiratory distress syndrome. JAMA 2009;302(17):1888–1895.
- Guérin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome (PROSEVA). N Engl J Med 2013;368(23):2159–2168.
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