Seattle Virus Watch families were observed, 1965–1969, for infections with paramyxoviruses and M. pneumoniae by agent isolation and antibody assay of serial sera. Infection rates, based on serology, exceeded those in Tecumseh where families contained fewer young children. Rates per 100 person-years were 44.4 for parainfluenzavirus, 21.6 for respiratory syncytial (RS) virus and 12.3 for M. pneumoniae. Preschool children experienced the highest rates for RS and parainfluenzaviruses but, for the latter, rates were also high among older children and adults. Within invaded families infection rates generally varied inversely with age, although for M. pneumoniae the rates for adults and 6–9-year old children nearly equalled the infant rate. The introducers' identity and/or the age-specific infection rates in invaded families support the role of young schoolchildren in community spread of M. pneumoniae and, together with older children and adults, of RS virus. Young schoolchildren were less important than infants, preschoolers, and adults in spreading parainfluenza viruses and less important than preschoolers and infants for mumps. The frequent infection of exposed older children and adults suggests that reinfection with all the agents studied is common. All agents spread significantly within families and secondary attack rates for the mostly non-immune infants indicated high infectivity of parainfluenza and mumps viruses. The basic high pathogenicity of these agents and of RS virus is indicated by the high frequency of illness among virus shedders (80–90%) and among sero-converting infants (≥68%). The less frequent illness of older persons with serologically proven infection is consistent with diminished clinical response to reinfection. Parainfluenza-associated illnesses were relatively severe and contributed up to 9.3% to total respiratory illnesses. RS virus-related illnesses also were severe but contributed less (4–5%) to total respiratory disease. Mumps-associated illness was largely respiratory, 65% overall, 77% in infected infants and 75% above age 9. Thus, mumps virus emerges as another respiratory pathogen which is spread largely by 2–5-year old children rather than by schoolchildren with “typical” parotitis.