Study participants. Eligible children were aged 2-11 y, in good health, and with no history of chronic lung disease. Eligibility was based on the inclusion/ exclusion criteria planned for a subsequent randomized trial of Echinacea. Thus, patients with a history of asthma, allergic rhinitis, autoimmune disease, or allergy to Echinacea or related species were not enrolled. Each participant was in the study until she/he developed a cold or completed a 120-d observation period without developing cold symptoms. Study data were collected from November 2007 to May 2008. The study was approved by the Seattle Children's Hospital Institutional Review Board and the Bastyr University Institutional Review Board. Written informed consent was obtained from the parents of study children; written assent was obtained from children aged 7 y and older.