For that matter, imagine something really UGLY but not necessarily fatal... Bring back measles, for instance... If they could get it disseminated widely enough (picture an urban transit bus system during rush hour - 50+ people sealed in a big box, coughing on each other), then watch the healthcare system melt down...
Our problem is that we think "high tech." They aren't. They don't build cruise missiles. They don't even steal them. They improvise.
And they won't repeat that either.