## INTRODUCTION AND EPIDEMIOLOGY

Bacterial and viral infections of the respiratory tract can result in a wide range of clinical syndromes including acute bronchitis, the common cold, influenza, and respiratory distress syndromes. Uniform definitions for the most common of these clinical syndromes are lacking because the symptoms associated with upper respiratory tract infections (URIs) frequently overlap and their causative pathogens are similar. The broad definition of acute bronchitis is as follows: a self-limited inflammation of the large airways characterized by cough without evidence of pneumonia, without an alternative medical disorder to explain the symptoms, or without a history of chronic lung disease.1

The common cold is a viral infection of the upper respiratory tract, primarily affecting the nasal mucosa, causing congestion, rhinorrhea, and sneezing. Influenza, or the "flu," is a respiratory illness caused by influenza viruses. Symptoms of influenza infection range from mild to severe and include fever, chills, myalgias, headache, malaise, cough, and fatigue. Severe acute respiratory syndrome is a unique respiratory illness that has clinical characteristics similar to other URIs but confers a high rate of mortality. Reported in 2012, Middle East respiratory syndrome coronavirus is novel viral respiratory infection that became a concern to the World Health Organization due to its fatality rate. Infections of the upper respiratory tract also cause specific clinical conditions like otitis media, pharyngitis, epiglottitis, bronchiolitis, laryngitis, tracheitis, and sinusitis (see corresponding chapters that discuss these diseases).

### EPIDEMIOLOGY

In ambulatory care settings nationwide, URIs are the third most common diagnosis.2 Annually, the estimated direct costs of noninfluenza viral URI in the United States is $17 billion, with indirect costs exceeding$22 billion.3 Acute bronchitis is among the most commonly diagnosed outpatient illnesses in the United States every year, with an annual incidence of about 5%,4 predominantly during fall and winter.1 The disorder accounts for approximately 10 million office visits per year, or 10 ambulatory visits per 1000 people per year. Symptom relief is the primary reason for office visits among adults within the first 2 weeks of illness, and many of these visits result in the unnecessary prescription of antibiotics by clinicians.5 The common cold afflicts adults two or three times every year, whereas children suffer up to eight colds annually.6,7,8 The incidence of colds caused by rhinovirus peaks during autumn months.9 Responsible for 22 million missed school days and 23 million lost work days, the common cold generates an enormous economic burden.4

Influenza affects millions of people worldwide every year during seasonal outbreaks (typically November through March in the northern hemisphere). An annual average of 41,000 Americans died from influenza infection from 1979 to 2001.10 The Centers for Disease Control and Prevention reported influenza-associated annual death rates between 1976 and 2007 that ranged from 1.4 to 16.7 deaths per 100,000 persons. Deaths associated with influenza have increased ...

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