Study design. At enrollment, parents completed a study form that included demographic information, use of daycare Ͼ 20 h/wk, school attendance, and exposure to cigarette smokers. In addition, a baseline nasal swab for PCR testing was collected. For patients with URI symptoms at the baseline visit, the baseline swab collection was rescheduled at least 14 d later. At enrollment, parents were given a cold symptom diary and asked to notify the research team as soon as they thought that their child was developing a cold.