The primary goal of this study is to collect and store serum and PBMC samples and obtain clinical and laboratory data from volunteers to determine in advance if they are potentially eligible to participate in future influenza challenge and/or vaccine studies. To accomplish this objective, up to 5000 participants will be enrolled in this protocol at the NIH Clinical Center clinic or day hospital in order to maintain a pool of participants who have been evaluated and can be screened for future influenza challenge studies.