It could have been an older disease that just hand't been recorded but did claim victims but nobody bothered to record the names of the victims.
Thus it went unnamed and eventually became forgotten but occasionally the rumors would pop up every now and then in the medical community.
They might have meant that the kids were the first ones able to be monitored/studied and confirmed to have to with in recent years.
Wait....how do they know that no one that has it will live past 12 if these two are the first ones with it?