This is what I found out about from the Better Health website, which I find helpful. Here's the link: http://www.betterhealth.vic.gov.au/bhcv2/b...Croup_in_detail
It says croup is caused by an infection to the throat and windpipe that is caused by a virus. From what I understand, the child catches a virus and croup is like a secondary infection caused by the virus. So, the virus is contagious but not all children who pick up the virus will develop croup.
In fact, the link above says:
The viruses that cause croup are very similar to those of the common cold. They start to be infectious with the first signs, such as a runny nose and cough, and remain infectious for up to five days. Only about one in 10 children who get these viruses will develop croup.
Hope this helps and your little one gets better quickly.