So. I used to work for a K-12 school district. 23 locations and over 9000 computers, all networked back to the central office. Either 5Ghz microwave links, gigabit fiber optic or our own leased line dsl. We had a couple localized outbreaks on old servers that couldn't be updated(NT4 server FTW), but it never spread from their sub-domain. I guess having a 90% Apple network helps as well.