It's treatable, but it's got to be treated early, when the symptoms are similar to flu, as I understand it. After a few days, the 'flu' goes away, you think you're better, then the full symptoms hit, by which time you're basically a dead man. As long as it keeps going in letters it probably can't do too much damage, because it can be isolated, and everyone who came into contact with it can be tracked down and treated, but if it were to be released on a large scale, the results would be catastrophic.