Viral Respiratory Infections

Acute respiratory infection is the most common illness of otherwise healthy people, and most of it is viral. Young children experience five to nine such illnesses a year, adults two or three. Most are trivial colds and sore throats, but respiratory infection is also a leading cause of childhood death in low- and middle-income countries, where viruses contribute to a large share of the millions of under-five deaths from acute respiratory infection each year. The two viruses that matter most for severe disease are influenza, which kills mainly the old, and respiratory syncytial virus (RSV), which kills mainly the very young.

Two ideas organise the whole topic. The first is the anatomical gradient. Six syndromes of increasing severity are described as the infection descends the tract: rhinitis (the common cold), pharyngitis, croup, bronchitis, bronchiolitis and pneumonia. This mapping is only a tendency, not a rule: the same virus can produce different syndromes in different people, even within one household, and a single syndrome can be caused by many unrelated viruses. Most respiratory viruses can cause disease at any level of the tract, though several have a predilection for one.

The second idea is immunity. Viruses whose infection stays confined to the respiratory mucosa, with little or no viraemia, provoke only a transient secretory immunoglobulin A (IgA) response and a weaker systemic one, so reinfection with the same or a drifted strain recurs throughout life. This contrasts with the viruses that enter by the respiratory route but then spread through the bloodstream to cause systemic disease, such as measles, rubella and varicella-zoster: these generate durable immunity and are treated under other topics. This topic covers the viruses whose disease is confined largely to the respiratory tract.

→ See Viral Respiratory Infections: an Overview for the respiratory syndromes, the viruses that cause each, their epidemiology and pathogenesis, and the natural history, clinical features and complications of infection from the upper tract down to the lung.

References and recommended reading

• Treanor JJ. Respiratory Infections. In: Richman DD, Whitley RJ, Hayden FG, editors. Clinical Virology, 4th edition, Chapter 2. Washington: ASM Press; 2016. The overview source for the respiratory syndromes, their viral causes and the clinical approach.
• Burrell CJ, Howard CR, Murphy FA. Viral Diseases of the Respiratory Tract. In: Fenner and White’s Medical Virology, 5th edition, Chapter 39. Academic Press / Elsevier; 2017. The concise account of the six-syndrome gradient and the syndrome-to-virus mapping.