The purpose of this study is to evaluate a new method of detecting the flu infection. This method may allow researchers to identify new proteins being made in response to an infection even before symptoms of the infection are present. The goal of this study is not to prevent the flu but to monitor the immune system response. The elderly and those with chronic health problems are at greater risk for complications (i.e., pneumonia, bronchitis [bacterial infection in the lungs], and sinusitis [bacterial infection in the sinuses]) from the flu. Early detection and diagnosis of the flu decreases the number of people with these complications. Participants will include healthy people between the ages of 21-40, between the ages of 60-89, or 90 years and older, who are living in the communities surrounding the 3 study sites in Virginia. There will be 5 study visits, and subjects will participate up to 1 month.