User blog comment:Sky-Dragoon-Twilight/Dragons' disappearance theories/@comment-10058473-20140913140247

I wouldn't be surprised if Skiadrum and Weisslogia just faked their deaths and fused with the dragon slayers instead, using the whole "sickness" and "kill me to gain power" to further disguise their disappearance

Thinking about it, dragons are resistant enough to not be killed easily even if they are not fighting back.