Evaluating Immune Response to Seasonal FluMist in Healthy Adults

Inclusion Criteria: * Medically eligible healthy men and women between the ages of 20 to 49 years old. Exclusion Criteria: * A medical condition that places them at high risk for complications from influenza, including those with chronic heart or lung disease, such as asthma or reactive airway disease; people with medical conditions such as diabetes or kidney failure; or people with illnesses that weaken the immune system, or who take medications that can weaken the immune system. * A history of Guillain-Barré Syndrome that occurred after receiving influenza vaccine * Currently pregnant * Have a severe allergy to chicken eggs or who are allergic to any of the nasal spray vaccine components * Have had flu within the current flu season. * Have had a flu vaccine within the current flu season. * Have close contact with someone whose immune system is so weak they require care in a protected environment (such as bone marrow transplant unit).