There are four groups in this study. Group 1 will consist of adults who will receive the higher dose of rB/HPIV3. Group 2 will consist of seropositive children who will be randomly assigned to receive the higher dose of rB/HPIV3 or placebo. Group 2 will not begin enrollment until the completion of Group 1 safety data review. Participants of both Groups 1 and 2 will be monitored for 10 days post vaccination for respiratory illness and for fever by self-reported temperature logs; these participants will be followed for a maximum of 28 days. Blood collection will occur at study entry and on Day 28; additional blood collection may occur up to 28 days prior to vaccination. Clinical assessments and nasal washes will occur at study entry and selected study visits. Group 1 participants will be contacted by phone on Day 180; Group 2 participants' parents or guardians will be contacted by phone on Days 1, 2, 8, 9, 11, and 180; study staff will ask about any illnesses or adverse events that may have occurred.