Acute respiratory infections (ARI) including influenza-like illness (ILI) can result in significant morbidity and mortality, direct medical expenses including outpatient visits and hospitalizations, and indirect costs due to lost work and school days. New population-based studies assessing the incidence and etiologies of ARI/ILI need to be conducted as ARI/ILI incidence and transmission as well as factors associated with ARI/ILI epidemiology have changed since previous population-based studies were conducted decades ago. Likewise, modern diagnostic techniques such as reverse transcription polymerase chain reaction (RT-PCR) expands the potential for greater knowledge regarding the epidemiology and etiology of viral ARI/ILIs; in previous historical studies in an estimated 75% of ARI/ILI-associate samples no virus was identified. Specific Aims: 1) Identify incidence of ARI and ILI in a representative population-based community sample; (2) Identify the etiology of these ARIs and ILIs; and (3) Assess individual and household factors associated with incidence and transmission of ARI/ILI. Secondary aims: (1) Assess differences in prevalence and etiology of ARI/ILI between this study's community population and those seeking acute care; and (2) Determine effectiveness of influenza vaccination on prevention of influenza illness in the study's community sample. Methods: Three hundred households will be followed for at least a year, maintaining a surveillance population of approximately 1000 individuals per year. Households will be identified by randomly sampling an existing cohort of households currently enrolled in a community-based survey study already recruited via a random, stratified sampling of households in the Washington Heights/Inwood area of Northern Manhattan. ARI/ILI surveillance will be conducted twice-weekly using text messaging with phone follow-up. Nasopharyngeal swabs will be obtained from ill household members and analyzed to identify ARI/ILI associated viruses via multiplex RT-PCR. Individual and household factors associated with ARI/ILI incidence and transmission will be assessed using general estimating equation regression models. Comparing this study's data with hospital data will assess differences in prevalence/etiology of ARI/ILI between community populations and those seeking acute care. Laboratory data will be combined with our pediatric/ adult immunization registry to determine seasonal influenza vaccine effectiveness against lab-confirmed influenza using a case-control analysis. Outcomes: The primary outcomes will be (1) incidence and prevalence of ARI/ILI; (2) etiologies of ARI/ILI; (3) factors associated with ARI/ILI incidence and transmission. Secondary outcomes will include (1) differences in the prevalence and etiology of ARI/ILI between community population and those seeking acute care; (2) influenza vaccine effectiveness in prevention of laboratory-confirmed influenza. If successful this project could provide important new information to help plan ARI/ILI interventions including new vaccines, vaccination strategies, and surveillance for viral pathogens, including future outbreaks.