In 75% of cases, the parainfluenza virus, mainly types 1 and 2, causes croup. Other viruses that can cause croup include influenza A and B, measles, adenovirus and respiratory syncytial virus (RSV). Spasmodic croup (croup with barking) does not have the usual signs of infection, such as fever, sore throat, and increased white blood cell count). Treating spasmodic croup is the same as treating regular croup.