The purpose of this study is to look at changes that occur in blood and nasal swab samples after a person's immune system responds to receiving a flu vaccine. The investigators will look at the immune response in 50 healthy volunteers, between 21 and 49 years of age, after they have been vaccinated with 1 dose of a flu vaccine sprayed into the nose. A blood sample and nasal swab of each nostril will be collected from volunteers before they receive the flu vaccination. After receiving the flu vaccine, volunteers will record health changes and daily temperatures on diary cards at home for 21 days. Blood will be drawn and nasal swabs will be collected from each volunteer at each of the 6 study visits. Volunteers will participate in the study for approximately 1 month. This study is being conducted at the Glennan Center for Geriatrics and Gerontology at Eastern Virginia Medical School.