respiratory infections associated with wheezing include respiratory syncytial virus (RSV), rhinovirus (RV), influenza, parainfluenza (PIV), adenovirus, and more recently identified viruses such as human metapneumovirus (hMPV) and human boca virus (hBoV). [11] [12] [13] [14] RSV causes epidemics of bronchiolitis and typically circulates in temperate climates during November to April with peaks in the winter months. [11, 15, 16] In tropical climates peaks are related to temperature and level of rainfall. [17] RSV infects the majority of children during their first year of life and essentially all children show evidence of RSV infection by age 3 years. [18] The initial RSV infection is typically the most severe, causing lower respiratory tract disease, such as bronchiolitis, in 20% to 30% of infants. [11, 18, 19] Other viruses such as RV, PIV, and adenovirus circulate nearly year-round with seasonal peaks of illness. [10, 11, 19, 20] Although RSV has long been identified as the major cause of infant bronchiolitis, the use of molecular techniques such as polymerase chain reaction (PCR) assays, has allowed for more sensitive detection of RV and other viruses in respiratory infections. [21, 22] RV circulates yearround with major peaks during the autumn and spring, is a leading cause of upper respiratory tract infections, and most children show evidence of having had an RV infection by age 2 years. [23] [24] [25] [26] [27] Although, RV historically was thought to be limited to the upper respiratory tract, investigations have demonstrated that RV can infect the lower airways, is associated with infant bronchiolitis, and becomes a more dominant pathogen in wheezing illness as children get older. [13, [28] [29] [30] 
Observational studies have described the viral etiology of bronchiolitis and/or wheezing illnesses in infants and very young children (Table 1) . [11, 13, 14, 19, 20, [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] The first descriptive studies of the viral etiology of bronchiolitis in the 1960-80s primarily used detection methods such as cell culture, antigen detection, and serologic testing. Kim and colleagues studied the epidemiology of RSV infection in infants and young children admitted to a children's hospital in Washington, D.C. from 1960 to the mid-70s and found that 40% of children with bronchiolitis had evidence of infection with RSV. [11] In a Norwegian study 1972-1979, the authors used immunofluorescence (IF) and/or cell culture to investigate the epidemiology of respiratory viruses in young children admitted to the hospital with respiratory illness. [31] Of the 979 infants diagnosed with a respiratory virus infection, RSV accounted for 58% of all diagnosed infections, and 87% of RSV infections were associated with lower respiratory tract illness. The study also described the typical distribution of known viruses at the time, including the winter epidemics of RSV, Influenza in the late winter and spring, and the seasonal distribution of RV with peaks in the autumn and spring. [31] Using multiple virus detection methods including PCR, Jartti and colleagues investigated the etiology of wheezing illness in 293 hospitalized children in Finland, September 2000 through May 31 2002 . Of the 76 infants studied, RSV (54%) was the most common virus detected, followed by picornavirus (42%) and HMPV (11%). [34] Calvo and colleagues studied consecutive respiratory admissions of 382 children less than 2 years of age to a single hospital in Spain from September 2003 to July 2005. In total, nasopharyngeal samples were obtained in 340 children. Among the 244/340 subjects (71.7%) in whom virus was isolated, RSV accounted for 41.5%, RV 34.8%, adenovirus 8.3%, influenza 6.5%, and hMPV 5.9%. In children in whom RV was detected, recurrent wheezing and bronchiolitis were the leading diagnoses. [36] Birth cohorts-Cohorts of children recruited at birth have allowed longitudinal follow-up of children, including those with less severe disease that did not require hospitalization. In the Tucson Respiratory study, a birth cohort of 1,179 infants enrolled May 1980 to January 1985, Wright and colleagues described the epidemiology of LRTIs during infancy. [38] Overall, 80% of infants were followed through the first year of life. In total, 348 children contributed 460 LRTIs evaluated by physicians, among which 292 respiratory cultures were obtained at the initial illness. The cumulative incidence rate of lower respiratory tract illnesses in the first year of life was 32.9/100 children. One percent of infants were hospitalized for their illness. IF and viral culture were employed to detect infection by RSV, PIV 1-3, influenza A and B, adenovirus, enterovirus, cytomegalovirus, and rhinovirus. An infectious agent was identified by viral culture in 193/292 (66%) of the available samples obtained from infants with lower respiratory tract illness. RSV accounted for 65% of the 183 first bronchiolitis diagnoses. [38] Other viruses detected in infants with bronchiolitis diagnoses included PIV 1-3 (14%), Influenza A and B (4%), and adenovirus (2%). An Australian cohort of 263 infants with at least one parent with doctor diagnosed atopy, recruited infants from July 1996 to July 1999 and followed them through the first year of life. [39] Nasopharyngeal aspirates and detailed information were collected prospectively during acute respiratory illnesses and PCR was used to identify viral respiratory pathogens. Acute respiratory illnesses associated with wheeze or "rattly chest" were classified as lower respiratory infections. Of the 329 lower respiratory infections, 28.9% were associated with wheeze. Rhinovirus was isolated in 45% of "wheezy" lower respiratory infections, and RSV in 16.8%.
Newly identified respiratory viruses-The role of newly identified viruses, such as hMPV and hBoV, in infant wheezing illnesses is still being defined. [12, 14, [41] [42] [43] Williams and colleagues investigated the role of hMPV in lower respiratory tract infections in children enrolled at birth and followed to age 5 years in a vaccine clinic. HMPV, first identified in 2001, was detected in 20% of 248 available samples obtained from children with lower respiratory tract illness in which no respiratory pathogen was previously detected. [12, 44] HMPV therefore accounted for 12% of lower respiratory tract illnesses in this cohort of otherwise healthy young children. [12, 44] Kesebir and colleagues used available respiratory specimens submitted to a hospital-based clinical virology laboratory (January to December 2004) to investigate the prevalence of hBoV, first identified in 2005. [14, 45] HBoV was detected in 5% of the study samples which were obtained from children less than 2 years of age and negative for other viruses, although testing for RV was not performed. Wheezing illness was associated with approximately 50% of the hBoV-associated cases. [14] Allander and colleagues found hBoV as the sole virus isolated in approximately 5% (12/259) of respiratory samples from children <3 years of age who were hospitalized with acute wheezing. [43] In general, the use of sensitive molecular techniques has confirmed the major role of RSV in infant bronchiolitis, broadened the role of viruses that were previously difficult to detect by culture, and allowed for the identification of new respiratory viruses. Furthermore, studies using PCR have demonstrated that while RSV appears to be the virus most commonly associated with wheezing in infants, RV plays a more prominent role after the age of 2 to 3 years. [30, 46] The increasing importance of RV in wheezing illnesses in older children-Epidemiologic studies in infants and children have highlighted the importance of RSVassociated wheezing in infants and RV-associated wheezing in older children. [30, 46, 47] Rakes and colleagues conducted a cross-sectional study of 70 children who presented to the emergency department with wheezing compared to 59 controls who presented to the ED with non-respiratory complaints, between January 1993 and April 1994. Respiratory viruses were isolated in over 82% of the wheezing children less than 2 years of age. RSV was the most common virus detected in children less than 2 years of age (68%) and was not detected in any control subjects. However, in the children less than 2 years of age, similar proportions of nasal aspirates from those with wheezing and controls were positive for RV (41%). In the children older than 2 years, viruses were detected in 83% of wheezing children. RV was detected by PCR in 71% of the wheezing older children compared to 36% of the non-wheezing controls. RSV was detected in 6% of the wheezing children who were 2 years or older. In addition, the authors found that 48% of the wheezing children who were 2 years or older had a positive test for RV and a measured marker of atopy compared to only 5% of the respective control group. [30] In a similar one-year study (2000) (2001) , 133 children (2 months to 18 years) admitted to the hospital for wheezing were compared to 133 age-matched controls admitted without wheezing. [46] In the younger children, virus was detected in 84% of the wheezing children compared to 54% of the respective controls. Consistent with other studies, RSV was the predominant virus in the younger children during the winter. However, RV was detected more frequently in young children hospitalized for wheezing during April through November. Among children older than 3 years, a respiratory virus was significantly more likely to be detected in children admitted for wheezing, than children without wheezing. RV detection was significantly associated with wheezing. In addition, wheezing was strongly associated with atopy, as measured by total IgE and skin testing, in the children older than 3 years. These studies highlight the different pathogens associated with wheezing illnesses by age and the association of RV and atopy with wheezing in children beyond infancy.
The association between bronchiolitis during infancy and the development of asthma has been an area of interest for decades. [4, [48] [49] [50] [51] [52] [53] [54] [55] [56] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] Most, but not all, previous studies have primarily included case infants who were hospitalized with bronchiolitis during infancy. Therefore studies examining wheezing only after hospitalization for bronchiolitis during infancy may not reflect the outcomes of the large majority of infants with bronchiolitis who have only outpatient visits, emergency department visits, or no healthcare visit at all. [76] Although several early studies focused solely on RSV bronchiolitis or were conducted before routine testing for RV was available, more recent studies have used PCR to investigate the association of non-RSV bronchiolitis and subsequent wheezing. [63, 67, 70, 71, 75, 77, 78] Therefore the diverse group of research investigations in this area includes case infants in whom the specific viral agents of bronchiolitis were not determined, case infants with only RSV bronchiolitis, or case infants with either RSV or non-RSV bronchiolitis. Overall, there is convincing evidence from several cohorts that RSV and RV bronchiolitis during infancy are risk factors or markers for subsequent wheezing within the first decade of life. [48, 56, [66] [67] [68] [70] [71] [72] 75] Hospitalization for bronchiolitis during infancy and the association with recurrent wheezing and asthma during childhood.
RSV bronchiolitis-As RSV is known to be a major cause of bronchiolitis during infancy, several early cohorts included case infants who were hospitalized with RSV bronchiolitis during infancy. [66, 71, 72, 79] Sigurs and colleagues studied the relationship between RSV hospitalization during infancy and asthma in a small cohort of Swedish children. [71, 72] This prospective study included 47 children hospitalized with RSV bronchiolitis during infancy and 93 matched controls. The authors defined the study outcomes as: "asthma" (3 or more episodes of bronchial obstruction verified by a physician), "recurrent wheezing" (3 or more episodes of bronchial obstruction not physician verified) and "any wheezing" (asthma, recurrent wheezing, or one or two episodes of wheezing). At age 7.5 years, approximately one-third of children with a history of severe RSV bronchiolitis were diagnosed with asthma and these children were significantly more likely to have a diagnosis of asthma than their non-hospitalized controls. [71] Despite the relatively small size of this cohort, at age 13 years the increased risk of asthma remained among children with a history of severe RSV bronchiolitis during infancy. [72] RSV bronchiolitis and non-RSV bronchiolitis-Historical data also demonstrate the increased risk of wheezing or asthma after non-RSV bronchiolitis and emerging data suggests that children with a history of LRTI with viruses other than RSV may have an even greater risk of subsequent wheezing. As early as the 1960s, in a study of hospitalized children less than 5 years of age, Simon and colleagues speculated that children with non-RSV bronchiolitis had a predisposition to develop asthma. [80] Murray and colleagues conducted an investigation of 73 children with either RSV or non-RSV bronchiolitis hospitalization during infancy and a retrospectively recruited non-hospitalized control group. [69] The authors found that the children hospitalized for bronchiolitis during infancy were more likely than controls to have wheezing (42.5% vs. 15.0%) at 5.5 years. [69] In addition, wheezing by parent report (34% vs. 13%) and use of bronchodilators (33% vs. 3%) at 9 to 10 years after the initial bronchiolitis episode were more common in children with a history of a bronchiolitis hospitalization. [48] Fjaerli and colleagues found that a group of 57 children hospitalized with bronchiolitis during infancy, whether RSV+ or RSV−, were more likely to be under a doctor's care for asthma at age 7 compared to a retrospectively recruited, non-hospitalized control group of 64 children. [68] Piippo-Savolainen and colleagues also found that children hospitalized for bronchiolitis in the first two years of life were more likely to have asthma in young adulthood. [81] In a subset of participants, Piippo-Savolainen and colleagues found that adults with a history of non-RSV bronchiolitis during the first two years of life were at greater risk of developing asthma than a comparison group with a history of RSV bronchiolitis. [64] In a cohort of 81 children, Kotaniemi-Syrjänen and colleagues investigated the relationship of non-RSV bronchiolitis during the first 2 years of life and the subsequent risk of asthma around age 7 years. [67] They found that an RV positive hospitalization for wheezing during the first 2 years of life was associated with a four-fold increased risk of asthma around age 7 years compared to non-RV hospitalizations. Finally, Garcia-Garcia and colleagues found an increased risk of early childhood asthma in children previously hospitalized with hMPV (23 children) or RSV (32 children) bronchiolitis compared to a control group hospitalized with gastroenteritis (38 children). [63] Overall, this data suggests that viral LRTI with viruses other than RSV are associated with as high or even higher risk of childhood asthma then RSV-associated LRTI.
Birth cohorts-There are a limited number of longitudinal investigations of viral infections during infancy and subsequent wheezing that have followed infants from birth, with the goal of prospectively identifying and investigating the spectrum of acute respiratory illnesses during infancy and early childhood on the risk of developing asthma. [70, 75, 82] These studies have allowed for the investigation of the association of viral LRTI that did not require hospitalization with subsequent wheezing. In the cohort of children enrolled at birth in the Tucson Children's Respiratory Study, Stein and colleagues found that children with a history of RSV LRTI in the first 3 years of life were 3.2 times more likely to have parental report of infrequent wheeze (1-3 episodes of wheezing in past year) and 4.3 times more likely to have frequent wheeze (>3 episodes of wheezing in the past year) at 6 years, compared to infants with no LRTIs in the first three years of life. [4, 70] However, the association of RSV LRTI during infancy and infrequent and frequent wheeze decreased with age and neither were significant at age 13 years. At age 13 years, 517 of the 888 children (58%) followed for the first 3 years of life were included. The authors suggested that although RSV LRTI during early childhood was a risk factor for recurrent wheezing, it was not a risk factor for atopic asthma.
Lemanske and colleagues found that an RV wheezing episode during infancy was the strongest predictor of persistent wheezing in preschool years among children enrolled in the Childhood Origins of Asthma Study (COAST). [75] The COAST cohort is different from the Tucson cohort in that it includes only children with an increased risk of developing asthma. [2] All of the children in the cohort have at least one parent with respiratory allergies and/or physician diagnosed asthma. The authors found that children with at least one moderate to severe RVassociated wheezing illness during infancy had a 6.6 increased odds of subsequent wheezing in the 3 rd year of life and those with RSV had a 3.0 increased odds of wheezing in the 3 rd year of life. [75] In the combined moderate-severe illness group without wheezing, there was an increased risk for wheezing in the third year of life (OR 3.9, 95% CI 1.1 -15). Data from this study is the first to show that, particularly in genetically susceptible hosts, even moderate to severe viral infections during infancy that are not associated with wheezing or hospitalization are associated with an increased risk of subsequent wheezing. [75] In another birth cohort of infants at high risk for asthma development, Kusel and colleagues found that children with a history of "wheezy" LRTI infections with RV or RSV during infancy were at increased risk of having wheezing at age 5 years. [82] 
A number of epidemiologic approaches have been employed to study the relationship between viral infections and asthma exacerbations ( Table 2) . [83] [84] [85] [86] [87] [88] [89] [90] [91] These approaches include comparing the prevalence of respiratory viruses detected in asthma patients with and without acute exacerbations, and comparing virus detection in patients with asthma to that in patients without asthma in community, emergency department, and/or hospital settings. In general, many studies prior to the use of sensitive molecular techniques, detected lower rates of viral infection during acute asthma exacerbations. [84, 88, 89, [92] [93] [94] More recently, the use of PCR has resulted in increased detection of respiratory viruses in patients with asthma exacerbations. [95] 
Viruses are important triggers of asthma exacerbations in children and have been detected in up to 80-85% of exacerbations in children in studies using PCR for viral detection (Table 2) . [30, 46, 84, 89, [95] [96] [97] Johnston and colleagues investigated the association of viral infections and asthma exacerbations in a 13 month longitudinal study of 108, [9] [10] [11] year old English children with reported wheeze or persistent cough. Families recorded twice daily peak flows and daily respiratory symptoms. Lower respiratory symptoms were defined and recorded as cough (day or night), wheeze (day or night), difficulty breathing/shortness of breath, or not fit to go to school because of chest problems. [95] Viruses were detected in approximately 80% of reported episodes of LRTIs with associated decreases in peak flow measurements. Picornaviruses, which include RV and enteroviruses, accounted for 2/3 of the positive samples. As a comparison, the authors tested respiratory aspirates for picornavirus from the group of 65 children who provided a respiratory sample when they were asymptomatic. The authors found that 12% of these samples were positive. In another investigation, Johnston and colleagues found strong correlations between the rates of upper respiratory tract infections, divided in half monthly segments, and rates of pediatric and adult hospital admissions for asthma as determined by ICD-9 codes. [98] These studies demonstrate the high prevalence of respiratory viruses in children with asthma and the correlation of peaks in respiratory infections with asthma hospitalizations.
The New Vaccine Surveillance Network (NVSN) is a population-based surveillance investigation of hospitalized pediatric patients ages birth to five years from two US counties. [13] Based on the 592 children enrolled October 2000 through September 2001, children with a history of wheezing or asthma had significantly higher estimated rates of RV-associated hospitalizations (25.3/1000 children) than those without a history of wheezing or asthma (3.1/1000 children), P<0.001).
with and without an acute exacerbation. [99, 100] Thumerelle and colleagues conducted a regional study of 82 French children (October 1, 1998-June 30, 1999), aged 2-16 years. In children with asthma, the authors found higher rates of virus detection among those hospitalized with an exacerbation compared to those without an exacerbation in the prior three weeks. [100] Khetsuriani and colleagues studied children aged 2 to 17 years with persistent asthma. Sixty-five children with acute asthma exacerbations and 77 children with well controlled asthma were enrolled. One or more viruses were detected in 63% of the patients with asthma exacerbations and in 23.4% of the patients with well-controlled asthma. RV was detected among 60% of case patients and 18% of controls. Symptomatic respiratory infections which were positive for at least one virus were associated with asthma exacerbations, while asymptomatic infections were not. [99] The September epidemic of asthma-Observational studies have also been used to investigate the association of respiratory viruses with asthma morbidity. An increase in asthma hospitalizations during early autumn has been noted in several countries and respiratory viruses, in particular RV, have been speculated as causative agents. [24, 91, [101] [102] [103] Johnston and colleagues investigated the etiology of the "September epidemic of asthma exacerbations" in a case group of 57 Canadian children with asthma who presented to the ED during the last 3 weeks of September compared to a group of 157 controls with asthma recruited from the community. [97] Although the control group did not have an ED visit, a majority reported asthma symptoms, including continuous or repeated breathing trouble, waking at night, and activity limitations. Viruses were detected in a significantly larger proportion of the children presenting to the ED than children who did not present to the ED (62% vs. 41%). Cases were also less likely to have been prescribed an inhaled corticosteroid (ICS) than controls. In a separate study, Johnston and colleagues used a mathematical model to investigate the relationship between peak asthma hospitalizations in Canada and the return to school. The authors found that over the 13 study years, the average timing of the peak of asthma hospitalizations in school age children occurred 17.7 days following the return to school, with later peaks for pre-school children and adults. The authors concluded that school age children were the likely source of the etiologic agent resulting in the yearly peak in asthma hospitalizations, with a plausible hypothesis being transmission of infectious agents such RV. [104] Asthma exacerbations in adults Respiratory virus detection in adults with asthma-Viruses are important triggers of asthma exacerbations in adults and studies using PCR have detected viruses in approximately 40-50% of exacerbations (Table 2) . [90, 92, [105] [106] [107] A study by Teichtahl and colleagues included adults admitted for asthma exacerbations and matched controls admitted for elective surgery, August 1993 to July 1994. Seventy-nine patients with asthma and 54 controls were included. Overall, 37% of the adults admitted with asthma had a virus detected compared to 9% of the control group. [105] Atmar and colleagues conducted a longitudinal study of 29 adults with asthma recruited from pulmonary clinics and a cross-sectional study of a convenience sample of 148 adults who presented to the ED with an asthma exacerbation. Viruses were detected using virus specific PCR. The authors found that in the longitudinal study, 44% of asthma exacerbation were associated with a respiratory tract viral infection. In the crosssectional ED study, 55% were associated with a viral infection. RV, coronavirus, influenza, and parainfluenza were the most common viruses detected. [106] Prevalence of virus detection in adults with asthma during exacerbations and quiescence-Several studies have performed viral detection both during asthma exacerbations and subsequent follow-up. Using PCR, Tan and colleagues investigated the prevalence of viral respiratory infections in 17 adults with near fatal asthma requiring ventilatory support, 29 adults hospitalized with an asthma exacerbation, and 14 hospitalized with COPD. [108] Samples for viral detection were taken during the acute asthma exacerbation and follow-up samples were obtained 4-6 weeks after hospital discharge. During the acute exacerbation, overall 52% of the samples were positive, including 59% and 41% of the near fatal asthma and acute exacerbations respectively. In the near fatal asthma group, 47% of the viruses detected were picornavirus and 24% were adenovirus. Viral detection was positive in 7% of the 29 specimens collected 4-6 weeks after hospital admission. [108] Other studies have used molecular diagnostic techniques to investigate the role of more recently discovered viruses in the pathogenesis of asthma exacerbations. Williams and colleagues determined the prevalence of hMPV in a cohort of 101 adults at initial enrollment during an asthma hospitalization (1999) (2000) (2001) (2002) (2003) and at follow-up 3 months later. [109] HMPV was detected in 6.9% of subjects at admission compared to 1.3% in follow-up. Furthermore, none of the subjects who were positive for hMPV at admission were positive at follow-up. [109] Another study involving this cohort of patients described the prevalence of RV in patients during an acute asthma exacerbation and 3-month follow-up. [110] Over the four year study period, 21% of the cohort was RV positive by PCR during the asthma exacerbation. Seventy-six of the 101 participants completed the 3 month follow-up. At follow-up only 1.3% (one patient) were positive and none of the subjects who were RV positive during the preceding asthma exacerbation were positive at follow-up. Subjects who were RV positive were more likely to smoke cigarettes and be a nonuser of ICS compared to RV negative subjects, similar to the findings of lower use of ICS among children seen in the hospital during the September asthma epidemic associated with RV among children. [97, 110] RV clinical lower respiratory tract infections in adults with and without asthma -Corne and colleagues conducted a longitudinal investigation of RV infection by following 76 subjects with asthma and their cohabitating partners without asthma over a 3 month period (September through December 1993). [111] Subjects maintained diaries of severity of upper and lower respiratory tract symptoms and nasal aspirates were obtained from subjects every 2 weeks. Overall, there were no differences in RV-positivity between the subjects with and without asthma. However, the authors found that participants with asthma had more frequent clinical lower respiratory tract infections associated with RV than controls, 43% vs. 17% respectively. In addition, the group of patients with asthma had significantly higher severity scores (median 1 vs. 0) and longer duration of illness (median 2.5 days vs. 0 days). [111] Summary Epidemiologic investigations have provided valuable insight into the role of respiratory viruses in wheezing illnesses in children and adults. Viruses are the most important cause of lower respiratory tract infections in infancy and early childhood, and LRTIs with respiratory viral pathogens have been identified as significant risk factors for the development of early childhood asthma. RSV is an important pathogen in wheezing illnesses during infancy and appears to become less commonly associated with wheezing illnesses in older children. The newly appreciated role of non-RSV LRTI and the strong association of RV illness with a marked increased risk of future wheezing among children born to a parent with asthma suggest a differential "asthmagenicity" of respiratory viruses in asthma pathogenesis. Although it is unclear whether respiratory viruses induce asthma development, children with severe infections during infancy are at increased risk of subsequent wheezing, and large longitudinal studies will hopefully help answer this critical question. Knowing whether respiratory viruses cause asthma presents the hope for a new strategy for asthma prevention. In addition, viruses are important triggers of asthma exacerbations in children and adults, being implicated in the vast majority of significant disease exacerbations, and respiratory viral illness prevention would likely decrease the significant morbidity related to this common chronic disease. 