Child w/Croup any danger to Asthmatic DownSyndrome Adult?

I have a 50 year old brother who has Down Syndrome and Asthma, and he has had pneumonia six times over the last 5 years. Two of our closest friends each have children who seem to contract Croup several times a year.  I know Croup is highly contagious among children.  HOWEVER, I cannot find any information as to whether Croup poses any danger to adults in compromised health such as Down Syndrome or Asthma.  I always make the two households "off limits" to us until the croup has passed, but always am made to feel by these two families that I am being overly cautious.

I'd really like to know once and for all if it does pose any danger to my brother, and if so, how long should I consider us "off limits" after a child has had an episode of Croup.   Does anyone know about this issue, or can you refer me to someone who might be able to answer this for me?  

Thanks so much for your help.