Respiratory Viral Infections

Respiratory viral infections are infections of the upper or lower respiratory tract caused by viruses, including influenza viruses, respiratory syncytial virus, rhinoviruses, parainfluenza viruses, human metapneumovirus, adenoviruses, and coronaviruses. They are among the most common human infections and a leading cause of acute illness, hospital admission, and death from lower respiratory infection worldwide.

Definition

Respiratory viral infections are acute infections of the respiratory tract caused by viruses, ranging clinically from self-limited upper-respiratory illness to severe lower-respiratory disease such as bronchiolitis and viral pneumonia.

Scope

This topic covers the principal respiratory viruses, the clinical syndromes they produce (from the common cold and pharyngitis to bronchiolitis, croup, and pneumonia), routes of spread, seasonality, and diagnosis. It is reference-educational; it describes the disease category and its epidemiology rather than offering individualized treatment.

Core questions

• Which viruses cause respiratory infection, and which clinical syndromes are characteristic of each?
• How do respiratory viruses transmit, and what drives their seasonal and pandemic patterns?
• Which patients are at greatest risk of severe lower respiratory disease?
• How are respiratory viral infections diagnosed and distinguished from bacterial causes?

Key concepts

• Upper versus lower respiratory tract infection
• Influenza viruses and antigenic drift/shift
• Respiratory syncytial virus and bronchiolitis
• Rhinovirus and the common cold
• Coronaviruses (seasonal, SARS, MERS, SARS-CoV-2)
• Droplet, aerosol, and contact transmission
• Seasonality and pandemic emergence
• Multiplex molecular diagnostics

Mechanisms

Respiratory viruses are typically acquired by inhalation of droplets or aerosols or by contact with respiratory secretions, then infect epithelial cells of the airway. Replication and the host inflammatory response produce mucosal injury, secretions, and, in the lower tract, alveolar damage and impaired gas exchange. Influenza viruses undergo continual antigenic drift and occasional reassortment (antigenic shift) that can generate pandemic strains, while coronaviruses can cross from animal reservoirs to humans and acquire efficient human-to-human transmission, as seen with SARS and related viruses (peiris-2003; cui-2018).

Clinical relevance

Respiratory viral infections are a leading reason for outpatient visits and a major cause of hospitalization, particularly bronchiolitis in infants and pneumonia in older adults and immunocompromised patients. Recognizing the viral nature of these syndromes informs appropriate diagnostic testing and infection control. This entry is descriptive and is not a guide to individual antiviral or supportive therapy.

Epidemiology

Respiratory syncytial virus is a leading cause of acute lower respiratory infection in young children, accounting for a substantial global burden of hospitalization (nair-2010), and seasonal influenza is associated with hundreds of thousands of respiratory deaths each year (iuliano-2018). Most respiratory viruses show winter seasonality in temperate climates, and several coronaviruses have produced large epidemics or pandemics after zoonotic emergence (peiris-2003; cui-2018).

History

The first human respiratory viruses were isolated in the mid-twentieth century with the advent of cell culture, beginning with influenza and followed by respiratory syncytial virus, parainfluenza, and rhinoviruses. The recognition of severe acute respiratory syndrome in 2003 (peiris-2003) and subsequent coronavirus outbreaks underscored the pandemic potential of zoonotic respiratory viruses (cui-2018). The reference synthesis of respiratory virology is Fields Virology (fields-virology).

Seminal works

• nair-2010
• iuliano-2018
• peiris-2003

Frequently asked questions

Are respiratory viral infections treated with antibiotics?

Antibiotics target bacteria and have no effect on the viruses that cause most respiratory infections; they are reserved for documented or strongly suspected bacterial complications, a clinical judgement beyond the scope of this reference entry.

Why do respiratory viral infections tend to peak in winter?

In temperate regions many respiratory viruses circulate most intensely in colder months, a seasonality attributed to a combination of environmental conditions, indoor crowding, and host factors, though the precise drivers remain an active research question.