Mrzzy, I don't know about actual figures over here but it is recognised that it is only a small minority of supporters who get involved and that there can be "professional troublemakers" involved. Still if you've got say a crowd of 20,000 and 5% turn nasty, you've got 1,000 people to deal with.

As for becoming normal, it's been part of UK football for as long as I can remember. My speculation is that it is less of a problem now over here than it has been in the past and I think the football clubs themselves take more of a stance against bad elements amongst their supporters. Lifetime bans from grounds for violent or hateful (eg. racist) behaviour aren't unheard of.