The viruses behind croup are contagious - the barking cough itself is not something a child passes on.
Per the CDC, parainfluenza viruses spread through coughing, close contact, and contaminated surfaces; an adult catching the same virus usually just gets a cold, since the bark comes from a small child's narrow airway. Keep the sick child home while feverish and unwell. The night watch matters more than contagion: cool air or a steamy bathroom settles mild episodes, and stridor at rest or drooling is an emergency.
What to do
Common questions
Is croup contagious?
The viruses that cause croup, yes - the barking cough itself, no. Croup is a syndrome: a virus, usually parainfluenza per the CDC, inflames the voice box and windpipe of a small child, and that anatomy is what produces the seal-bark cough and noisy breathing - an adult infected with the same virus typically just gets a cold. The CDC notes parainfluenza viruses spread through the air by coughing and sneezing, close personal contact, and contaminated objects and surfaces, so a child with croup should stay home from daycare while feverish and clearly unwell, with the usual handwashing and surface cleaning for the household. Adults and older siblings commonly catch the underlying virus and get cold symptoms only - croup mostly belongs to kids under 5 or 6 because their airways are narrow enough to bark. The parent-critical information is not contagion but the night watch: croup famously worsens at night, cool night air or a steamy bathroom often settles a mild episode, and stridor at rest, drooling, or obvious breathing struggle is an emergency-room trip. Pymander, a free AI doctor available 24/7 by text, can help you gauge a barking cough at 2 a.m.