The discharge diagnosis and supporting clinical parameters of the infant acute respiratory illness visit were reviewed to confirm whether each child had lower respiratory tract infections (LRTI) or URI (n = 628) or another diagnosis (n = 46). LRTI and URI were defined using both the physician discharge diagnosis, as well as post-discharge chart review, and those cases that were not clearly identified as either LRTI or URI were reviewed by a panel of paediatricians who determined whether the illness represented an LRTI, URI, croup or other, which included those that could not be categorized with the available clinical information. Acute respiratory illness severity was determined using the ordinal bronchiolitis score that incorporates admission information on respiratory rate, flaring or retractions, room air oxygen saturation and wheezing, into a score ranging from 0 to 12 (12 being most severe). 27, 28 highly sensitive and specific qRT-PCR assays for many common respiratory viruses, including hMPV, HCoV, RSV, influenza A and B, parainfluenzaviruses 1-3 and rhinovirus. Real-time RT-PCR is performed using the Cepheid Smart Cycler II. All specimens are first tested for GAPDH to confirm integrity of RNA and monitor for potential PCR inhibitors. Negative and positive controls are included with each run, including RNA runoff transcripts to generate a quantitation curve. [14] [15] [16] [17] [18] [19] [20] [21] If rapid antigen and/or culture for RSV or Influenza were performed at the discretion of the admitting physicians during the index visit, these results are also captured and entered in the database. The cells are collected using Mid Turbinate Peds Nylon Flocked Swabs (MicroRheologics, Brescia, Italy), and placed into collection and digestion media, followed by processing and plating onto collagen IV coated tissue culture and grown at 37°C in a 5% CO2 incubator. To develop techniques for isolation, short-term culture, and in vivo modelling of epithelial and stromal cells, a xenograph model has been developed. Following short-term growth in culture, cells are transferred to denuded rat tracheas and implanted subcutaneously in nude mice.