The common cold is the most common reason that patients visit physicians. More lost school days occur because of the common cold than all other causes combined. The common cold is the most frequent upper respiratory tract infection (URI). Wald et al studied the usual duration of community-acquired viral URI and the incidence of complications (otitis media/sinusitis) of these URI in infancy and early childhood. Children in daycare were found to be more likely to have protracted respiratory symptoms than children in home care. The mean duration for a single URI ranged from approximately 7 days (for 1-2 year olds in home care) to approximately 9 days (for children younger than 1 year old in daycare). The percentage of simple URIs lasting more than 15 days ranged from 6.5% (for 1-year-old to 3-year-old children in home care) to 13.1% (for 2-year-old to 3-year-old children in daycare). Symptoms of the common cold include sneezing, rhinorrhea, cough, mild sore throat, congestion, low-grade fever, and malaise. Acetaminophen, oral fluids, rest, and saline nose drops with gentle suction are the only treatments generally recognized as safe and partially effective. Symptoms usually abate in 5 to 7 days. When a child begins to recover and develops localized pain such as earache, chest, throat or facial pain, increased irritability or lethargy, decreased oral intake, high fever, a new purulent nasal discharge, difficulty breathing, or night and day cough, complications such as pharyngitis, sinusitis, otitis or lower respiratory tract infection should be suspected. The real challenge is to distinguish between recurrent or persistent colds that need more time and complications that require medical intervention.
Cough can persist after viral URI for several reasons. A patient occasionally may develop a dry, tic-like, irritative cough that may be present for days or weeks. Presumably the viral infection has irritated the airways and stimulates the cough receptors. Passive or active smoke inhalation may stimulate the cough receptors further. Removing environmental irritants and the passage of time may be the only modalities effective for this difficult cough.
After the URI, the next most common cause of chronic cough is caused by asthma. Cough variant asthma is an occult form of asthma in which the only sign or symptom is chronic cough. It is a problem among all age groups that may go unrecognized. A trial of oral or inhaled bronchodilators with a spacer in young children and a metered-dose inhaler in older children is warranted in patients with or without wheezing and persistent cough after a laboratory evaluation excludes other, more complex, albeit less common, causes. Oral bronchodilators are considered for those children too young to use a spacer or for those for whom the cost of a home nebulizer would be prohibitive.
As many as 5% of acute URIs are complicated by an acute infection of the paranasal sinuses. Persistent nasal discharge and chronic cough usually are caused by infections of the ethmoid or maxillary sinuses. These sinuses are present at birth, with frontal and sphenoid sinuses forming later. Inflammation from infectious and allergic processes may lead to obstruction of the sinus ostia and predispose to secondary bacterial infection. Drainage from the sinuses stimulates the cough receptors. The most common organisms for bacterial sinusitis are Streptococcus pneumoniae, Moraxella catarrhalis, and Haemophillus influenzae 
 Bronchitis is a term used by parents and physicians. Unfortunately, this is a nebulous term without a clearly defined set of signs or symptoms. The term does not imply a specific cause, a treatment, or a prognosis. Because a significant degree of overlap exists between colds and asthma, the term bronchitis generally should be avoided and replaced with a more specific diagnosis. Because URIs and asthma have causes other than bacteria, antibiotics usually are not indicated.
Most chronic coughs fall into one of the three aforementioned categories: persistent or recurrent URI, reactive airway disease, or sinusitis. Despite the frequency of these diagnoses, other less common causes need to be remembered to avoid inappropriate reassurance or treatment. By considering the age of the child and the clinical circumstances, precise diagnosis can be achieved. 
In the infant younger than 3 months of age, the triad of staccato cough, tachypnea, and conjunctivitis should raise the question of chlamydia pneumonia. Tuberculosis or fungal infections may be seen in children and present as a chronic, hoarse cough thought to be caused by airway compression secondary to enlarged perihilar or paratrachial nodes. Pertussis starts with 2 weeks of mild rhinorrhea and cough before progressing to the characteristic paroxysms of cough or posttussive vomiting. Loud, brassy cough following a viral URI associated with attention seeking behavior and "la belle indifference" may indicate a psychogenic or habitual cough. Foreign body aspiration should be considered at every age, especially in the toddler. Even an esophageal foreign body can produce a chronic cough by pressing on the posterior pharynx. Overfeeding, a dysfunctional swallowing mechanism, gastroesophageal reflux and bottle propping can cause chronic aspiration, thereby producing chronic cough. Other less common causes of chronic aspiration are congenital abnormalities such as tracheoesophageal fistula, laryngeal cleft, and adductor vocal cord paralysis.
Immune deficiency diseases can present with prolonged respiratory symptoms or multiple pneumonias. HIV has become an important cause of immune deficiency in children. Unusual opportunistic infections such as Pneumocystis carinii pneumonia can be seen, but the most common manifestation of HIV disease in children is recurrent bacterial infection such as otitis media or pneumonia that usually responds temporarily to a standard antibiotic regimen. Lymphocytic interstitial pneumonia can complicate HIV infection and may persist with a nonproductive chronic cough, respiratory distress, failure to thrive, and pulmonary infiltrates.
Inability to mechanically clear mucus from the respiratory tract may cause trapping of foreign material and lead to chronic cough. Patients with cystic fibrosis present with cough, failure to thrive, and recurrent lower respiratory tract infections often secondary to pseudomonas. These patients produce a thick, tenacious sputum that is difficult to clear from the airway. In the immotile cilia syndrome, an abnormality in the mucocilary apparatus results in frequent respiratory infections.
Other less common congenital problems causing chronic cough include any condition in which a large, aberrant, thoracic structure compresses the airway. Examples of this include anomalous great vessels, bronchogenic cysts, or pulmonary sequestration. A simple URI can produce respiratory compromise in these conditions in addition to tracheobronchomalacia in which the central airways are soft and collapse more easily.
Few published reports detail the psychogenic cough. It generally is not thought of as rare, however. This cough is characterized as croupy and explosive. It never occurs during sleep and is not affected by antitussives. It usually is regarded as a respiratory tic and can be precipitated by various stimuli such as school phobia, attention seeking, or anxiety.
Cough is a reflex response of the respiratory tract to irritants in the form of mechanical, inflammatory, chemical, or thermal stimuli. Dust, smoke, and foreign bodies are common types of mechanical stimuli, whereas extrinsic compression from tumor, lymph nodes, or enlarged anatomic structures is uncommon. Inflammation activates cough receptors because of hyperemia and edema of the mucous membranes. Organic compounds as well as acidic and basic vapors irritate receptors in the smaller airways. Inhalation of hot or cold air may stimulate cough where there is an underlying pathologic condition of the respiratory tract.
A reflex is a reproducible, neurally mediated response to a stimulus. In general, the components of a reflex arc include the peripheral sensory receptor, afferent pathway, cough center, and motor response unit. From the respiratory tract, the afferent pathway is mainly via the vagal and superior laryngeal nerves. The cough center is in the medulla. The afferent pathway is mainly via the phrenic and intercostal nerves. The motor unit is the diaphragm and intercostal musculature. The larynx, carina, and bifurcations of large bronchi are especially sensitive to mechanical stimuli, whereas the terminal bronchioles and alveoli are more sensitive to chemical stimuli. Interestingly, cough receptors also are found in the pharynx, external auditory canal, and stomach. If cerumen impinges on the eardrum, a chronic cough can be triggered and will persist until the canal is disimpacted of the cerumen. 
Air is inspired into the lungs. The air then is entrapped in the lungs by the closure of the epiglottis and the vocal cords. The abdominal muscles contract quickly and forcefully against the diaphragm in conjunction with contraction of the internal intercostal muscles. The pressure within the lungs rises sharply. An explosion of air occurs when the epiglottis and vocal cords open.
The common cold is a mild and self-limited syndrome caused by a viral infection of the mucosa of the upper respiratory tract. The symptoms are well known and include sneezing, rhinorrhea, sore throat, and cough. The rhinovirus and the coronavirus cause up to 45% of all colds. Assorted other viruses such as influenza, parainfluenza, respiratory synctial virus, and adenovirus account for another 10% to 15%. Transmission appears to be via respiratory droplets and hand-to-hand contact. Self-inoculation then occurs on the conjunctival or nasal epithelium. The respiratory mucosa becomes hyperemic and edematous. With the onset of inflammation, the cough receptors are stimulated and the reflex is initiated.
Asthma is characterized by airway obstruction that is reversible. Reduction in airway diameter is caused by smooth muscle contraction, epithelial inflammation, and mucous secretion into the lumen. Here too, the inflammation triggers cough receptors. 
Production of cough in sinusitis is via two mechanisms. The postnasal drip into the pharynx associated with sinusitis serves as a mechanical stimulus, whereas the hyperemia of respiratory epithelium serves as an inflammatory stimulus.
The history is of importance in determining the cause of persistent cough. Attention first should be directed at the symptom itself, eliciting duration and descriptive qualities of the cough as well as associated symptoms. Indicators of serious illness should be identified next. Identifying the age of the child will help to narrow the differential diagnosis. A more general history then may be obtained, including a prenatal and feeding history in the case of an infant. Growth and development, childhood illnesses, exposures, operations, injuries, hospitalizations, immunizations, medications, allergies, and psychosocial and family history also are necessary. Access to medical care for the child and the health belief system of the family, when evaluated, often will elicit enlightening and useful information.
Determining the duration of the cough is critical in sorting out persistent cough from recurrent cough. A frustrated parent may present with their child stating "Johnny has been coughing all winter!" On questioning, many of these chronic coughs will turn out to be a series of separate URIs not caused by a single pathophysiologic process.