Croup Reaction to the Pneumococcal, and Influenza vaccines according to the manufacturer. A viral disease, often caused by respiratory syncytial virus (RSV), which results upper respiratory symptoms such as a runny nose and a barky (seal-like) cough. Episodic difficulty in breathing can be quite dramatic in some cases. Attacks of croup are manifest by a loud noise when the individual inhales, known as stridor. In most cases, breathing humidified air (for example steamy bathroom) clears the symptoms of acute croup. Treatment is generally conservative with a vaporizer and fever control. (27 Sep 1997)