[Jan W]

My understanding is that the different species carry micro-organisms that are normal for their species and do not cause them any infections. In another species, the micro-organisms are potentially harmful and therefore called pathogenic - ability to cause disease.

As soon as they hatch they are exposed to micro-organisms and some will be the harmless ones that reside on them. They do not live in a sterile but hopefully a clean environment.

I don't know the particular micro-organisms and would be interested to know if anyone has this detail.

I do know for example that chickens happily carry salmonella, but in a human they are pathogenic with the potential to cause humans infections.

Also, you may need to ask if different species have different requirements in terms of temperature and humidity, otherwise one species could receive an advantage over the other.

However, please note i am not an expert and look forward to others to comment.