Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Viral upper respiratory infection and influenza
Infectious diseases, microbiology and sexual health · UK scope · Sources checked 27 Aug 2026 · Clinical review pending
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Time-critical presentation
Escalate respiratory distress, hypoxaemia, shock, confusion, inability to maintain hydration, severe chest pain, cyanosis or rapid deterioration. High-risk patients with suspected influenza need prompt antiviral assessment without waiting for test confirmation when treatment is indicated.
Overview
Distinguish self-limiting viral upper-respiratory infection from influenza and bacterial or lower-respiratory complications, using targeted antiviral treatment and safety-netting rather than routine antibiotics.
Upper-respiratory viral infection is a clinical syndrome of nasal obstruction or discharge, sore throat, cough and systemic symptoms caused by numerous viruses. Influenza sits on the same differential but has greater potential for systemic illness, pneumonia and deterioration in vulnerable people. COVID-19 cannot be separated reliably from symptoms alone.
Management begins with severity and risk rather than pathogen curiosity. Most low-risk patients need symptom care and safety-netting. Testing is valuable in hospitals, outbreaks, high-risk treatment windows and when infection-control action changes. Antivirals are targeted to influenza likelihood, timing, severity and risk group using current seasonal UKHSA guidance.
Clinical worsening needs a new diagnosis. Bacterial pneumonia, sinus or ear infection, myocarditis, encephalopathy, asthma or COPD exacerbation and dehydration can follow a viral illness. The correct response is examination and targeted investigation, not automatic extension of an antiviral or addition of antibiotics.
Key points
Most acute coryzal illnesses are viral and improve with supportive care; purulent nasal discharge alone does not prove bacterial infection or justify antibiotics.
Aetiology
01
Respiratory viruses
Rhinovirus, seasonal coronaviruses, influenza, respiratory syncytial virus and other viruses spread through respiratory particles, hands and contaminated surfaces.
02
Host susceptibility
Older age, pregnancy, immune suppression and chronic cardiac, pulmonary, renal, hepatic or neurological disease increase influenza complication risk.
03
Seasonal and exposure factors
Winter circulation, household crowding, healthcare outbreaks and incomplete current vaccination increase the probability of infection and transmission.
Pathophysiology
1
Mucosal epithelial infection
Viruses enter susceptible nasal, pharyngeal or respiratory epithelial cells, replicate and disrupt local barrier and ciliary function.
2
Innate inflammatory response
Interferons and cytokines produce congestion, sore throat, fever, myalgia and malaise while recruiting antiviral immune cells.
3
Lower-airway extension
Influenza and selected viruses can reach bronchi and alveoli, causing pneumonitis, impaired gas exchange and respiratory failure.
4
Secondary bacterial susceptibility
Epithelial injury and altered immune defence allow pneumococcus, staphylococci and other bacteria to cause pneumonia after initial viral illness.
Clinical features and red flags
Uncomplicated viral upper-respiratory infection
Rhinorrhoea, nasal congestion, sore throat and cough with preserved breathing, hydration and alertness usually support self-limited upper-airway disease.
Influenza syndrome
Abrupt fever, marked myalgia, headache, malaise and cough during circulating influenza increases probability, although atypical presentation occurs in older or immunosuppressed adults.
Lower-respiratory complicationRed flag
Dyspnoea, hypoxaemia, pleuritic pain, focal crackles or recurrent fever after initial improvement suggests viral pneumonitis or secondary bacterial pneumonia.
High-risk hostRed flag
Pregnancy, immune suppression, frailty and chronic organ disease lower the threshold for testing, antiviral treatment and same-day clinical review.
Investigations
01Respiratory viral PCR or validated molecular testFirst step
Why
Identify influenza, SARS-CoV-2 or another virus when the result changes treatment or infection control.
Interpretation and limitations
Sensitivity depends on illness timing and collection. Detection can persist and coinfection occurs, so match the result to the syndrome.
02Pulse oximetry and full observations
Why
Detect lower-respiratory involvement or physiological deterioration.
Interpretation and limitations
Normal resting saturation does not explain severe breathlessness automatically; exertional or serial change may reveal evolving disease in selected patients.
03Chest radiograph
Why
Assess focal pneumonia, diffuse pneumonitis or another thoracic complication when lower-respiratory signs are present.
Interpretation and limitations
Imaging is unnecessary for routine coryza. An early normal film may not exclude evolving pneumonia when physiology deteriorates.
04FBC, renal function and inflammatory markers
Why
Assess complication, dehydration and treatment modifiers in moderate or severe illness.
Interpretation and limitations
Biomarkers cannot reliably distinguish bacterial from viral infection alone. Renal function determines oseltamivir adjustment.
Differential diagnosis
01
COVID-19
Symptoms overlap substantially; current molecular testing and epidemiology rather than a single symptom distinguish SARS-CoV-2 when the result changes care.
02
Bacterial pneumonia
Focal consolidation, purulent lower-airway sputum, recurrent fever and bacterial microbiology support bacterial complication rather than uncomplicated upper-respiratory disease.
03
Streptococcal pharyngitis
Prominent tonsillar inflammation with validated clinical criteria and absence of viral features may support a bacterial sore-throat pathway.
04
Non-infectious airway disease
Allergic rhinitis, asthma, reflux and pulmonary embolism can cause cough or breathlessness without a primary acute viral infection.
Management
01Low-risk upper-respiratory illnessUse supportive care and safety-nettingFirst stepSymptoms remain confined to the upper airway with stable observations, hydration and no high-risk host factor.+
1Explain the likely viral course, review analgesic contraindications and advise fluids, rest, hand hygiene and respiratory etiquette.
2Avoid routine antibiotics and low-value testing, while considering COVID-19 or influenza testing when current public-health or treatment rules make it useful.
3Give a specific review trigger for breathlessness, dehydration, confusion, chest pain, recurrent fever or deterioration after initial improvement.
02Likely influenza in a high-risk patientStart the antiviral decision promptlyInfluenza is clinically or microbiologically plausible and severity, admission or risk factors meet current treatment guidance.+
1
Complications
01
Viral pneumonitis
Lower-airway inflammation may produce hypoxaemia, diffuse infiltrates and acute respiratory distress requiring admission and ventilatory support, especially in high-risk adults.
02
Secondary bacterial pneumonia
Damaged epithelium and altered immunity permit new consolidation, sepsis, pleural infection or necrotising disease after apparent initial improvement.
03
Cardiac or neurological injury
Influenza can precipitate myocarditis, encephalopathy, seizure, stroke or decompensation of chronic cardiac disease, causing disability beyond the respiratory illness.
04
Exacerbation of chronic disease
Asthma, COPD, heart failure and diabetes may destabilise during infection and require disease-specific treatment alongside antiviral care.
Monitoring and follow-up
Advise the patient to monitor breathing, hydration, alertness and trajectory rather than temperature alone, with a clear route for urgent reassessment.
For admitted or high-risk influenza, trend oxygen need, observations, renal function and ability to absorb oral treatment.
Review new focal chest findings, recurrent fever or inflammatory deterioration for bacterial pneumonia and culture before antibiotics when safe.
Check oseltamivir dose after any renal change and confirm intended course length in immune suppression or severe disease with specialists.
Report and manage institutional clusters through infection prevention and health-protection pathways, including staff and bed-movement implications.
Special situations
Sputum colour is not proof
Neutrophils can make secretions yellow or green during viral illness, so colour alone does not establish bacterial infection.
Influenza can be afebrile
Older and immunosuppressed adults may present with confusion, weakness or respiratory decline without a dramatic temperature.
Timing matters but severity matters more
Antivirals work best early, yet hospitalised or severe eligible illness may still warrant treatment beyond a simple community time window.
Second deterioration is a clue
Initial improvement followed by new fever, dyspnoea or focal pain should trigger assessment for bacterial or inflammatory complication.
Common pitfalls
01
Prescribing antibiotics because nasal discharge or sputum is coloured.
02
Excluding influenza because fever is absent in a frail adult.
03
Waiting for PCR before treating severe eligible influenza.
04
Using oseltamivir without renal-dose review.
05
Attributing new hypoxaemia to a simple upper-respiratory infection.
Practice
Two practice questions
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Question 1 of 2
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Infectious diseases, microbiology and sexual healthOriginal SBA
Antibiotic decision in coryza
A healthy adult has four days of rhinorrhoea, sore throat and cough, stable observations and no focal chest signs. Nasal discharge is green. What is the best management?
Sources and review status4 sources · checked 27 Aug 2026 · clinical review pending+
Sources
Sources and review status
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Influenza more often causes abrupt fever, myalgia, headache, profound malaise and dry cough, but clinical features overlap with COVID-19 and other respiratory viruses.
Assess severity and host risk: pregnancy, older age, immune suppression and chronic cardiac, respiratory, renal, hepatic or neurological disease increase complication risk.
Use respiratory testing when it changes antiviral, isolation, outbreak or admission decisions; a negative result is interpreted against timing and sample quality.
Offer oseltamivir according to current NICE and UKHSA treatment criteria, ideally early, and do not delay in hospitalised or severely ill eligible patients solely for a result.
Antibiotics do not treat uncomplicated influenza or viral upper-respiratory infection; new focal consolidation, recurrent fever or clinical decline suggests bacterial complication.
Advise hydration, appropriate analgesia, infection-control measures and clear red flags; review vaccination after recovery rather than during the acute diagnostic decision.
05Bacterial cultures when complicated
Why
Identify secondary bacterial pneumonia or sepsis before antibiotics when feasible.
Interpretation and limitations
Sputum quality and prior treatment affect yield; collect blood cultures for severe systemic illness without delaying indicated antibiotics.
Assess onset, pregnancy, immune status, renal function, swallowing and exposure and obtain respiratory testing if it will not delay indicated treatment.
2Start the current first-choice antiviral at the correct renal-adjusted dose and use infection-control precautions; seek specialist advice for severe immune suppression or treatment failure.
3Reassess for pneumonia, bacterial superinfection and organ complications if improvement does not occur as expected.
03Clinical deteriorationLook below the upper airwayNew hypoxaemia, focal signs, shock, confusion or recurrent fever develops during or after the viral syndrome.+
1Perform ABCDE, repeat observations and examine for pneumonia, asthma or COPD exacerbation, myocarditis, pulmonary embolism and dehydration.
2Obtain chest imaging and targeted blood or microbiological tests, start sepsis or pneumonia treatment when criteria are met and continue indicated antiviral therapy.
3EscalationAdmit or escalate according to physiology and host risk, with a defined review of bacterial evidence and antimicrobial duration.
Key medicines and prescribing safety2 treatments · regimens, roles and cautions+
Treats suspected or confirmed influenza in eligible at-risk, admitted or severely ill patients.
Oseltamivir
Usual adult treatment is 75 mg orally twice daily for five days, with renal adjustment and specialist extension for selected cases.
Adjust to current renal function and review vomiting and neuropsychiatric symptoms. Pregnancy is an influenza risk state and is not, by itself, a reason to withhold indicated treatment; use current UKHSA pregnancy guidance and seek specialist advice for severe immune suppression, complicated disease or treatment failure.
Relieves fever, headache, sore throat and myalgia during uncomplicated viral illness.
Paracetamol
Use 500 mg to 1 g orally up to four times daily, maximum 4 g in 24 hours for most adults.
Reduce maximum exposure in low body weight, liver disease, malnutrition or heavy alcohol use and check combination cold remedies to prevent inadvertent overdose.