Human metapneumovirus (HMPV) is a virus that can cause respiratory illness. In older adults, those with asthma, infants, and children, illness can be severe, but in healthy adults the virus frequently causes no symptoms. The National Institute of Allergy and Infectious Diseases (NIAID) is working to develop a vaccine for HMPV that could be given to infants. Before potential vaccines can be tested, information about how HMPV affects healthy adults is needed. This study will examine the effects of exposure to HMPV in healthy adults.