what causes croup
Croup is usually caused by viruses. It is most often caused by the parainfluenza virus, but it can also be caused by several other viruses including the flu viruses (influenza A and B). Bacteria sometimes also cause croup.roup is usually caused by viruses. It is most often caused by the parainfluenza virus, but it can also be caused by several other viruses including the flu viruses (influenza A and B). Bacteria sometimes also cause croup.