No one knows.

One school of thought is that extraterrestrial pathogens could pose limited risk. Earthbound pathogens have co-evolved with humanity in order to harm us better, but xenobiological pathogens would not have this advantage and might not pose much of a threat.

On the other hand, it's entirely possible that the random chances of evolution could create a xenovirus that's airborne in humans and bootstraps a fatal prion disease in everyone it infects.