 The goal of the study is to capture the complete history of exposure towards microbial challenges in the first six years of life and to study interactions between different components of microbial exposure as well as effects on the development of immunity. The investigators apply a diary in which parents document symptoms of respiratory and gastrointestinal infections of their child, and collect nasal swabs and stool samples obtained on first day of symptoms. In addition, the investigators collect nasal swabs and stool samples once per year during asymptomatic period. The investigators intend to collect venous blood from children participating in the study at the age of six years. In a subsample called intensive subcohort, the investigators collect asymptomatic probes three monthly during the first two years of life and obtain venous blood of the children at the age of one and two years. 