@pdworkin: It’s really the same reason pit bulls are illegal in some places, a few bad apples. But as long as you handle your ferret as a baby, and continue to play and handle them throughout their life the chance they’ll be a biter is slim, and even then, you have to be downright negligent to allow a testy ferret to be handled by anyone but yourself. I have never had a ferret that was a problem. I had one who wasn’t handled for a few weeks, and she developed some cranky issues, but in the same amount of time being back with me she was back to normal.
As for diseases, again, as long as your responsible this should not be an issue. Just don’t let them out without a harness on. Even if it’s around your house. One of mine got out when I was eight, Emily the Ferret made it all the way down the street before a neighbor caught her under a cooking pot, called the authorities and had her put down within an hour. She knew it was mine, keep a tag and a collar on your pets.