Acute bronchitis in children is usually of viral origin and is self limiting. Severity of bronchitis clinically is assessed by whether the baby has fast breathing, is uncomfortable, not accepting feeds, is turning blue. Bronchitis in very small babies specially newborn babies or babies who are 14-15 days old can be more severe because there bronchial airway are very small and they tend to have complications more often. It is safer to keep these babies in the hospital for observation and to monitor for any complications. It can be fatal if the baby has any of the danger signs mentioned above. But dont be under the impression that all babies with bronchitis will have a very severe disease as gets reflected from your mail.
Another fact is that 14-15 days old babies usually don't get bronchits commonly. We should assess whether what we are labelling simply as bronchitis is not a reflection of any other disease.