Monto, A. S. (Dept. Epidem., Sch. Public Health, Univ. Michigan, Ann Arbor, Mich. 48104). The Tecumseh study of respiratory illness. V. Patterns of infection with the parainfluenzaviruses. Am J Epidemiol 97: 338–348, 1973.—Patterns of parainfluenzavirus infection during the 6 years of the Tecumseh study of respiratory illness were examined. Type 3 was isolated most frequently and type 4, least. Although most isolates were recovered from children under the age of 10 years, 23% of the strains were isolated from older individuals. The type of illness associated with virus isolation appeared to be more severe than that encountered with rhinovirus infections. Type 3 was endemic in occurrence; type 2 was limited to relatively short periods of prevalence but the pattern of type 1 isolation changed during the study. Types 1 and 3 were isolated most frequently in the fall, and type 2 in winter. Testing of 2819 sera collected routinely from persons on surveillance confirmed the periodicity of infection with types 1, 2 and 3. Infections with type 3 clearly occurred more frequently early in life than types 1 and 2. For the latter two types, serologic infection rates were highest in the 5 to 9 year age group. The rate per person-year for the entire population studied was 28.6% with types 1, 2 and 3 combined. It is, therefore, conduded that these infectious agents are among the most frequently encountered respiratory pathogens of man.