3. eMedicine - Bronchitis : Article by Samuel Ong, MD
Although bronchitis refers to inflammation of the trachea and bronchi, other segments of the respiratory tract may also be involved because acute bronchitis usually occurs in relation to the common cold or other respiratory illness.
http://www.emedicine.com/emerg/topic69.htm

4. Common Cold
With kids getting as many as eight colds per year or more, this contagious viral infection of the upper respiratory tract is the most common infectious disease in the United States and the number-one reason children visit the doctor and stay home...
http://kidshealth.org/parent/infections/bacterial_viral/cold.html

5. MedlinePlus: Viral Infections
Viral Infections Viruses are capsules with genetic material inside. Viral infections are hard to treat because viruses live inside your body's cells. Antibiotics do not work for viral infections. Viral Infections -- see more articles.
http://www.nlm.nih.gov/medlineplus/viralinfections.html