The common cold is generally a mild illness of the upper respiratory tract, affecting the nasopharynx and paranasal sinuses. Rhinoviruses, which cause more than 40 percent of colds, and coronaviruses, which cause 10 to 20 percent, are the most important pathogens. RSV, parainfluenza virus, influenza virus, adenovirus, and some enterovirus infections also contribute. Nasal discharge is usually clear initially but may become purulent.
Symptoms tend to be more prominent in the morning.
Physical findings are nonspecific and include nasal discharge and mucosal erythema. Lower respiratory tract illness and fever are more common in young children. Practical diagnosis is usually based on clinical findings. The differential diagnosis includes allergic rhinitis, bacterial nasopharyngitis in infants, and, in rare instances, nasal diphtheria or bacterial nasal infection in adults. Because nearly 50 percent of colds last longer than one week and 25 percent last up to two weeks, it is often unclear whether a complicating bacterial sinusitis has occurred.