Clinical Information

Respiratory infections are common and generally cause self-limited illnesses in healthy, immunocompetent hosts. Viruses account for a significant percentage of respiratory diseases, but bacteria can be associated with respiratory infections. Although respiratory illnesses are frequently mild, viruses may cause significant morbidity and mortality in immunocompromised hosts (eg, transplant recipients, patients with underlying malignancies).

Influenza viruses (types A and B) and respiratory syncytial virus (RSV) are 2 common causes of viral respiratory illness, with peak incidence in the winter and spring months in the Northern hemisphere. Both viruses can cause a clinically indistinguishable syndrome characterized by fever, cough, headache, and general malaise. RSV is a leading cause of respiratory illness in young children. Early diagnosis of influenza and RSV is important so necessary infection control precautions can be taken if the patient is hospitalized, and antiviral therapy can be considered if the patient is hospitalized or considered at high-risk for severe disease.(1) Human metapneumovirus is also a cause of respiratory illness in both children and adults.

Human rhinovirus and coronavirus serotypes HKU1, NL63, 229E, and OC43 are the causative agents of the common cold, with symptoms including runny nose, sore throat, and malaise. Infections with rhinovirus and coronaviruses are extremely common, due to the large number of serotypes of these viruses. Most infections are mild and self-limiting; however, immunocompromised individuals may suffer more severe illnesses, including lower respiratory tract disease.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an RNA virus that causes COVID-19. Like other coronaviruses that infect humans, SARS-CoV-2 can cause both upper and lower respiratory tract illness. Symptoms can range from mild (eg, the common cold) to severe (eg, pneumonia) in both healthy and immunocompromised patients. SARS-CoV-2 transmission occurs primarily via respiratory droplets. During the early stages of COVID-19 disease, the symptoms may be nonspecific and resemble other common respiratory infections, such as influenza.

Parainfluenza viruses and adenovirus are also common causes of viral infection, especially in young children. Parainfluenza viruses are most common during the spring, summer, and fall months, with symptoms including fever, runny nose, and cough. However, parainfluenza viruses may also cause more severe lower respiratory disease, such as croup or pneumonia. Adenoviruses may infect a range of organ systems, with sequelae ranging from cold-like symptoms (sore throat) to pneumonia, conjunctivitis (pink eye), or diarrhea. Similar to the viruses described above, parainfluenza viruses and adenoviruses generally cause mild, self-limited infections but may cause severe disease in immunosuppressed patients.

Respiratory infections may also be caused by bacterial pathogens, including Bordetella pertussis, Bordetella parapertussis, Chlamydia pneumoniae, and Mycoplasma pneumoniae. B pertussis is the causative agent of pertussis, or whooping cough, a disease characterized by a prolonged cough that may be associated with an inspiratory whoop and post-tussive vomiting. B parapertussis causes a similar, but generally less severe, illness. M pneumoniae is a cause of upper respiratory infection, pharyngitis, tracheobronchitis, and pneumonia. C pneumoniae is a rare cause of pneumonia.