The basic system is that when you have bees, you should learn to identifye diseases when they appear.
Swarms are normally rid off diseases. They carry of course mites (tracheal + varroa) but you can clean them with thymol and formic acid.
If the sick hive is in the corner of the yeard, it does not protect other hives in the yard.
But this is the first time when I see an advice that "dont take swarms because they have diseases".