medicated feed or not

I need to decide whether or not to give medicated feed to my chicks or not.
I want to raise them organically and I know that you can't give them medicated feed if you want to do that.
But how bad are the risks?
If you have a flock that you hatched or got at a day old and you are not bringing in other chickens can they still get sick?
Are these illnesses just caught from other chickens?
I heard that they can also catch diseases from birds that fly overhead,true or not?
If you give them medicated feed, how long are you suppose to do that for?
thanks for any input.