Acute respiratory illness in the community. Frequency of illness and the agents involved.

Investigations of respiratory illnesses and infections in Tecumseh, Michigan, USA, were carried out in two phases, together covering 11 years. During the second phase, there were 5363 person-years of observation. Respiratory illness rates in both males and females peaked in the 1-2 year age group and fell thereafter. Adult females had more frequent illnesses than adult males; illnesses were less common in working women than in women not working outside the home. Isolation of viruses fell with increasing age; rhinoviruses were the most common isolate. Influenza infection rates, determined serologically, suggested relative sparing of young children from infection with type A (H1N1) and type B. Infection rates were highest in adult age groups for type A (H3N2). The isolation and serological infection rates were used to estimate the extent to which laboratory procedures underestimated the proportion of respiratory illnesses caused by each infectious agent; data from other studies were also used in this estimation. Severity of respiratory illnesses was assessed by the proportion of such illnesses that resulted in consultation of a physician. Rhinoviruses produced the greatest number of consultations. Overall, physician consultations were associated with 25.4% of respiratory illnesses.