"It's not the first time that an undead has managed to infect people, either to safely feed upon them or to control them. This kind of trick, however, always followed the same pattern.
"The tissues would be hidden inside their victim and act as a parasite. Hence you would find two energy signatures, each belonging to a different being and then remove the unwanted one.
"In our case, the Draugr tissues form a symbiote that fuses with its host, making it impossible to treat them separately. That's why it managed to fool us for so long. It followed the pattern of Jiera's plague but had a different effect, leading us to believe it was simply a different strain of the disease." Marth said.
"That's it!" Quylla jumped off her chair in excitement. "Professor, if I recall correctly, Jiera's plague deadly trait was its ability to bond with its victim tissues and make them rot from the inside so that any attempt to heal it would just hasten the decay process."