Secondary outcomes: Up to 2 weeks after the acute respiratory tract infection, severity of symptoms (cough, wheeze, noisy breathing, breathlessness, rhinitis) will be assessed on a 4-graded scale (0-3, mild 1, moderate 2 and severe 3) by the parents as well as exact number of bronchodilator puffs used (symptom diary 1). Up to 2 months after the acute respiratory tract infection, parents will record days with rhinitis, cough, noisy breathing, wheezing, night-time symptoms, medication (bronchodilator, corticosteroid or antibiotics) and visits to physician or hospitalizations for wheezing (symptom diary 2). Up to 12 months, the number of out-patient care visits and hospitalizations for wheezing and courses of systemic or inhaled corticosteroids will be recorded (diary 3). Multiple immunologic markers and pulmonary function after 24-36 months and at age 6 years will be measured.