There is several poultry dieses that the chickens will get but not Kill them or they may not even appear to have it. They can pass these to the wild birds which usually will be killed by the same dieses. So it is always safer to keep a lot of separation between chickens and other fowl.
But then again some have had no problems.
The advice on keeping them separate comes from State and Federal AG departments which have the large producers in mind, where an undetected dieses would create a large finacial lose.
As long as you have not had a problem with your current flock, there is a good chance you won't have a problem. I can recall many farm yards with a lot of different fowl in them from my younger days.