There is evidence that the pediatric population, in particular, may play an essential role in influenza transmission. One recent study showed that children between the ages of 3 and 4 years were consistently the first population group to be affected by influenza illness over several winter seasons. In addition, influenza illness in children less than 5 years of age was the strongest predictor of mortality among adults. Collecting samples from adults as well as children in the pre-school age group would enable researchers to further examine the specific role this population plays in the evolution of influenza virus lineages and the development of dominant strains. In this prospective observational study, the researchers are therefore proposing to collect samples from pediatric patients at Children's Hospital and from adults at Beth Israel Deaconess Medical Center. This will enable the researchers to compare the clinical manifestations resulting from infection by similar strains and to examine differences in the genomic sequences of the viruses between the two population groups. The primary objective of this study is to conduct a pilot study to explore correlations between influenza viral strains and clinical characteristics of infected patients, including vaccination status, organ system involvement, and disease severity. With regard to outcome measures, the researchers will determine rates of clinical features associated with specific viral strains. The clinical characteristics the researchers will examine are vaccination status, illness symptoms (presence of fever, cough, runny nose/ stuffy nose, headache, myalgia, fatigue), initial O2 saturation, treatment with antibiotics or antivirals, hospitalization, daycare or kindergarten attendance, exposure to elderly persons, and length of illness. The secondary study objective is to conduct a pilot study to compare the influenza genomic sequences and clinical data from pediatric and adult patients infected with influenza and to explore potential viral transmission patterns between these two populations. With regard to the outcome measures, the researchers will determine rates of clinical features and of specific viral strains in the pediatric and adult populations. The clinical characteristics the researchers will examine are the same as in the primary objective. The researchers will also identify any temporal patterns in the infections caused by specific viral strains in the two groups. Specifically, the researchers will examine whether any viral strains first emerge in the pediatric population prior to infecting the adult group. A total of 500 patients with respiratory illnesses will be recruited at Children's Hospital Boston and at Beth Israel Deaconess Medical Center. At Children's Hospital, patients 30 days to 5 years of age will be recruited; at Beth Israel Deaconess Medical Center patients of any age will be eligible. The researchers will recruit patients of both genders and of any health status. Respiratory samples and clinical information will be collected from each participant. It is hoped that this study will help further the researchers' understanding of how influenza viruses evolve, spread, and cause disease. In the future, this may help in the development of more effective vaccines and prevention measures, as well as treatments for people infected with influenza.