Viral illnesses are the most common cause of upper respiratory symptoms. Symptoms of a viral illness often appear over several hours without prior illness. Common viral illnesses include colds and the flu (influenza).

Bacterial respiratory tract infections are less common than viral infections. Bacterial infections may develop after a viral illness such as a cold or the flu. Bacterial infections may affect the upper or lower respiratory system, and symptoms tend to localize to one area.

The most common sites of bacterial infections in the upper respiratory system are the sinuses and throat. Pneumonia is the most common bacterial infection of the lower respiratory system.

Antibiotics are effective against bacterial infections only after they have developed, so most doctors do not prescribe them until a bacterial infection is suspected or confirmed.