No what she had is not said to be the unknown disease just hinted, but the unknown disease is arsenic poisoning, due to in reality steeles green dye, the thing is and i didn't know this they even ate food with it, seems wierd clothing dye and paint dye being used in food, but the cookies might have had it in, differant intake blanche its clothes while abigail its food could explain differant symptoms added to adult vs child. though as to symptoms remember its not shown but she could have been vommiting n such and its not said out of respect simply ment by she has the unknown disease, after all noble commits suicide they would say its something else to save face, nobility be wierd like that.