BACKGROUND:There is a large, unexplained variation in the frequency of childhood infections. We described incidence and risk factors of infections in early childhood.METHODS:Simple infections were captured during the first 3 years of life in the Copenhagen Prospective Studies on Asthma in Childhood 2000 birth cohort. Environmental exposures were analyzed by quasi-Poisson regression and sparse principal component analysis.RESULTS:The 334 children experienced a median of 14 (range 2–43) infectious episodes at ages 0 to 3 years. The overall rate of infections was associated with the number of children in the day care (adjusted incidence rate ratio [aIRR] 1.09 [1.2–1.16]) and the m2 per child in the day care (aIRR 0.96 [0.92–0.99]). Upper respiratory infections were also associated with the number of children in the day care (aIRR 1.11 [1.03–1.20]) and the m2 per child in the day care (aIRR 0.95 [0.91–0.99]), whereas lower respiratory infections were associated with caesarean section (aIRR 1.49 [1.12–1.99]), maternal smoking (aIRR 1.66 [1.18–2.33]), older siblings (aIRR 1.54 [1.19–2.01]), and the age at entry to day care (aIRR 0.77 [0.65–0.91]). The sparse principal component analysis revealed a risk factor profile driven by tobacco exposure, social circumstances, and domestic pets, but could only be used to explain 8.4% of the infection burden.CONCLUSIONS:Children experienced around 14 infections during the first 3 years of life, but incidences varied greatly. Environmental exposures only explained a small fraction of the variation, suggesting host factors as major determinants of infectious burden.