The respiratory syncytial virus is the most common cause for bronchiolitis.

Other viruses and bacteria that may cause bronchiolitis include:

• Adenovirus
• Chlamydia trachomatis
• Echovirus
• Influenza
• Mumps
• Mycoplasma pneumoniae
• Parainfluenza
• Rhinovirus

In bronchiolitis, the virus or bacteria invades and lives inside the cells that line the bronchial passages. This leads to airway inflammation with bronchoconstriction and increased mucus production. The smaller airways can become plugged from excess mucous, resulting in difficulty breathing.