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Background Although multiple respiratory viruses circulate in humans, few studies have compared the incidence of different viruses across the life course. We estimated the incidence of outpatient illness due to 12 different viruses during November 2018 through April 2019 in a fully enumerated population. Methods We conducted active surveillance for ambulatory care visits for acute respiratory illness (ARI) among members of Kaiser Permanente Washington (KPWA). Enrolled patients provided respiratory swab specimens which were tested for 12 respiratory viruses using reverse transcription polymerase chain reaction (RT-PCR). We estimated the cumulative incidence of infection due to each virus overall and by age group. Results The KPWA population under surveillance included 202 562 individuals, of whom 2767 (1.4%) were enrolled in the study. Influenza A(H3N2) was the most commonly detected virus, with an overall incidence of 21 medically attended illnesses per 1000 population; the next most common viruses were influenza A(H1N1) (18 per 1000), coronaviruses (13 per 1000), respiratory syncytial virus (RSV, 13 per 1000), and rhinovirus (9 per 1000). RSV was the most common cause of medically attended ARI among children aged 1–4 years; coronaviruses were the most common among adults aged ≥65 years. Conclusions Consistent with other studies focused on single viruses, we found that influenza and RSV were major causes of acute respiratory illness in persons of all ages. In comparison, coronaviruses and rhinovirus were also important pathogens. Prior to the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), coronaviruses were the second-most common cause of medically attended ARI during the 2018/19 influenza season. [ABSTRACT FROM AUTHOR]