Check this out: http://arstechnica.com/news.ars/post/20050921-5337.html
"Players of Blizzard's incredibly popular World of Warcraft are reporting the outbreak of a virtual plague that is spreading across major cities in the virtual land of Azeroth, infecting player characters at an alarming rate.
The trouble started when Blizzard programmers added a new instance, which is a separate area connected to the outside world that players can enter and attempt unique quests. One of these instances, Zul'Grub, contained the god of blood, Hakkar. Hakkar was a powerful foe that could cast spells of his own, including a spell called Corrupted Blood. This spell did a large amount of damage to any player within the vicinity of the casting, and the effects lingered on after the spell was over."
An interesting impact of "connected systems" 😉