Table Causes of Common Viral Respiratory Syndromes Syndrome Common Causes Less Common Causes Bronchiolitis RSV Influenza viruses Parainfluenza viruses Adenoviruses Rhinoviruses Common cold Rhinoviruses Coronaviruses Influenza viruses Parainfluenza viruses Enteroviruses Adenoviruses Human metapneumoviruses RSV Croup Parainfluenza viruses Influenza viruses RSV Influenza - like illness Influenza viruses Parainfluenza viruses Adenoviruses Pneumonia Influenza viruses RSV Adenoviruses Coronaviruses (including SARS-CoV-2) Parainfluenza viruses Enteroviruses Rhinoviruses Human metapneumoviruses RSV = respiratory syncytial virus.