Health,
1.) I’m not making any assumptions. Why would you assume that for 30 years the disease was only circulating in certain populations? Most people didn’t know if they had it at that time as more and more people got educated about it more and more cases were noticed everywhere. Also even just “intravenous drug users” is way too big of a group to quarantine in any case. That group alone would include an incredible amount of people, how do you think they would track down anyone who had used an intravenous drug in the previous 10 years??
2.) Outbreaks start with 1 person. 1 person can expose hundreds of people in just a few hours (just has to take a plane and then an airport shuttle etc.) by trying to block off half a continent and not trying to stop the epidemic over there it is virtually guaranteed that people will slip through the cracks eventually, the only difference then will be that the virus would have had more time to mutate.