How long is parainfluenza / croup contagious?

At least a week. Most viruses follow the same basic pattern. Viral numbers expand exponentially until our immune systems can generate an effective response (fever is the first attempt). Antibody response to viruses takes 3-5 days and peak infectivity is usually the first week. We continue to shed virus for 1-2 weeks after. Respiratory viruses are spread by droplets which can travel up to 4 feet through the air.
Contagious. Croup caused by virus spread with air droplets.Signs of respiratory infection, such as a runny nose or cough.Barking noise or stridor from swelling in airway /larynx.Croup lasts five or six days and highly contagious in children between the ages of 3 months and 6 years (the average age is 2 years),Adults can become infected with same virus that caused their child’s croup usually less symptoms.
1-4 weeks. Immunocompetent children infected with parainfluenza virus may shed virus for 1-3 weeks even after symptoms subside.