The purpose of this study is to measure the immune response (how the body fights infection) to an experimental preparation of live Respiratory Syncytial Virus (RSV). A better understanding of this virus may be useful in development of vaccines and treatments. Participants will include 20 healthy adults age 21-40. Study procedures will include drawing blood, urine samples, respiratory exams, vital signs and temperature, diary cards, nasal mucus weight and nasal washes and swabs. All participants will receive vaccine via nose drops. Patients will participate in the study for about 2 months.

Respiratory infections are a major cause of morbidity and mortality in the elderly. Recently, respiratory syncytial virus (RSV) has been shown to account for a significant portion of these illnesses. The causative factors associated with severe RSV disease in the elderly are not well understood. The contribution of viral load, host inflammatory responses and the cellular immune response to disease pathogenesis are unknown. A better understanding of these processes may be useful in the development of vaccines and therapeutics for RSV disease. Although the adult groups at highest risk for serious RSV disease are elderly and high-risk adults, we propose to use the challenge model in young healthy adults to determine optimal timing of specimen collection and the feasibility of performing certain tests before moving ahead to the target groups with natural infection. The purpose of this study is to measure the immune response (how the body fights infection) to an experimental preparation of live Respiratory Syncytial Virus (RSV. Participants will include 20 healthy adults age 21-40. Study procedures will include drawing blood, urine samples, respiratory exams, vital signs and temperature, diary cards, nasal mucus weight and nasal washes and swabs. All participants will receive vaccine via nose drops. Patients will participate in the study for about 2 months.

Inclusion Criteria:

Healthy adults ages 21 to 40 years. Able to and has given informed consent to participate. Negative urine pregnancy test, if female. Pre-challenge serum neutralizing titer to RSV A2 by microneutalization (MNA) of \