It's not a decease, it was a curse.

According to Marsella, spells weren't as complicated as others thought they were. Usually, when a person casts a spell, the caster could undo it. However, there were spells — especially ancient forbidden curses — that even the caster couldn't undo. That didn't mean there was nothing to be done. It only needs a powerful — much more powerful than the caster and a capable witch to undo a spell. 

Maleficent. 

A witch that was said to be the most powerful witch to have ever existed could cast and dispel any curse. Since the Grimsbanne were originally vampires, there were certain things they couldn't do. They weren't born witches and only learned witchcraft in hopes they could deal with their own cursed blood. 

In other words, Marsella and Sunny's best bet was Aries, and that power lies within her. After all, those two were almost in the same dilemma. The only difference was that Sunny didn't wish to die.