This is a nonrandomized, non-comparative study conducted in 50 healthy subjects, between 21 and 49 years of age, who have been vaccinated with 1 dose (0.5 mL) of Influenza Virus Vaccine Live, Intranasal FluMistTM, a live trivalent nasally administered vaccine. The central hypothesis is that immune responses to vaccination can be quantified by proteomic profiling of serum and nasal proteins, and that host responses to different infectious agents are unique and can be "fingerprinted" by proteomics. Using influenza virus vaccination, this study proposes to use mass spectrometry platforms to profile and characterize proteins from serum samples obtained from recipients. These samples will be used to develop a proteomic profiling system for monitoring vaccine response, and will eventually allow earlier detection/diagnosis of infection. Annually, natural influenza infections cause serious international public health problems that are particularly severe in elderly people, who account for more than 90% of influenza mortality. The current trivalent split virus influenza vaccine is cost-effective, but its efficacy is greatly reduced in the elderly population. The long-term goal of the investigators' approach is to develop tools useful for reducing the morbidity and mortality of influenza from natural and potential bioterrorism-related infections by improving measures of vaccine efficacy and early diagnosis. This study is being conducted at the Glennan Center for Geriatrics and Gerontology at Eastern Virginia Medical School. Solicited adverse events (AEs) and daily temperatures will be collected on diary cards for 21 days. Unsolicited AEs will also be collected throughout the duration of the study. Blood will be drawn and nasal swabs will be collected from each subject at each of the 6 study visits. Subjects will participate in the study for approximately 1 month. The primary objective of this study is to analyze serum samples before and after FluMist intranasal attenuated live-virus influenza vaccination to identify surrogate markers reflective of the immune response by using surface-enhanced laser desorption/ionization-time of flight (SELDI-TOF) and matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry proteomic profiling tools reflective of the immune response. Concurrent T-cell activation, cytokine assays, and hemagglutination inhibition (HI) serologic assays will correlate cellular and humoral responses to influenza vaccination with protein profiling changes. Proteins obtained from intranasal swabs pre and post vaccination will be analyzed. The investigators anticipate identifying multiple host response proteins from this analysis using combinations of gel electrophoresis, MALDI-TOF and tandem MS/MS approaches. The secondary objective is safety. Data on solicited reactions will be collected from Day 0 to Day 14. The time of occurrence of AEs will be used to determine if there is a correlation with the time of increase in certain protein biomarkers. The primary endpoint is to identify surrogate markers reflective of the immune response in serum and/or nasal proteins and to correlate these markers to cellular and humoral responses (demonstrated with T-cell activation, cytokine assays, and the hemagglutination inhibition [HI] serologic assays) to influenza vaccination. The secondary endpoint is safety. Data on local reactions and solicited systemic reactions will be collected from Day 0 to Day 14 and categorized according to severity. Unsolicited AEs will be collected and categorized throughout the study by severity, duration, and relatedness to vaccine. The time of occurrence of AEs will be used to determine if there is a correlation with the time of increase in certain protein biomarkers.