Table 2.

Clinical characteristic of infectious agents triggering asthma

Pathogen	Clinical symptoms	Mode of transmission	Specific immunity
Rhinovirus	Most common virus causing upper respiratory illnesses (40% to 50%)	Person-to-person contact	Some type-specific immunity; of variable degree and brief duration; generally offers little protection against other serotypes
Coronavirus	A common cause of URI in adults and children; also implicated in lower respiratory tract infections.	Person-to-person via aerosol or fomites	Cellular and humoral immunity are required for virus clearance. Re-infections seem to occur throughout life (implying multiple serotypes [at least four are known] or antigenic variation).
	The superficial layers of the nasal mucosa temperature (32°–33°C) yields optimal growth.
Influenza virus	Systemic involvement differentiates from other viral illness, with fever being almost always present. The onset is abrupt with marked malaise and myalgias.	Person-to-person via droplets, direct contact or contaminated nasopharyngeal secretions	Specific antibodies confer immunity.
			Antigenic serotypes (A, B, and C) are subclassified by the presence of two surface antigens, hemaglutinin (HA) and neuroaminidase (NA). Antigenic shifts are determined by major changes in HA or NA with emergence of new virus strains, leading to epidemics or pandemics. Antigenic drifts are minor changes with variations within subtype, continuously resulting on variant viruses and leading to seasonal epidemics.
Adenovirus	Adenovirus most commonly causes respiratory illness, but, depending on the infecting serotype, other illnesses may occur; half of infections are asymptomatic.	Via respiratory secretions through person-to-person contact or via the oral-fecal route	There is a worldwide distribution, with a higher prevalence in developing countries and in lower socioeconomic groups. Generally, by school age, most children have been exposed to various serotypes.
Parainfluenza virus	Major cause of laryngotracheobronchitis (croup). Commonly causes URI, pneumonia, and bronchiolitis. Exacerbates symptoms of chronic lung disease.	Person-to-person via direct contact or contaminated nasopharyngeal secretions through respiratory tract droplets and fomites	Reinfection usually causes a mild illness limited to the upper respiratory tract. Most people have exposure to all serotypes by 5 y of age.
Respiratory syncytial virus	Causes acute respiratory illness in patients of all ages.	Humans are the only source of infection. Transmission occurs by direct or close contact with contaminated secretions. Good hygiene habits are important because the virus may persist in environmental surfaces for many hours and on the hands for 30 min or more.	Almost 100% of children are infected with RSV by 2 y of age.
	It is the most common cause of bronchiolitis and pneumonia in infants.
Human metapneumovirus	Varied — includes cough, coryza, fever, irritability, anorexia, wheezing, pharyngitis, vomiting, or diarrhea	Unknown	By 5 y of age nearly 100% individuals have been infected
Mycoplasma pneumoniae	Most commonly causes respiratory illnesses such as acute bronchitis, including pharyngitis, and occasionally otitis media, which may be bullous. Ten percent of infected individuals develop pneumonia within a few days that may last for 3–4 wk.	Causes disease only in humans; it is highly transmissible by droplets.	Epidemics occur every 4–7 y because immunity is not long lasting.
		The long incubation period (ranging from 1–4 wk) along with long asymptomatic carriage (for weeks to months) facilitates familial spread, which may continue for months.
Chlamydia pneumoniae	Responsible for a variety of respiratory diseases including pneumonia, acute bronchitis, and, less commonly, pharyngitis, laryngitis, otitis media, and sinusitis. Many infected patients are asymptomatic or mild to moderately ill. A prolonged illness may be present with cough persisting for 2–6 wk, sometimes with a biphasic course.	Assumed transmission is person-to-person, via infected respiratory secretions.	Recurrent infection is common, especially in adults.
			In tropical, less-developed areas, infection seems to occur earlier in life. In the United States, 50% of adults have antibodies by 20 y of age, with initial infection peaking between 5 and 15 y of age.