I don't think it outside the realm of possibility that Zahra may have been sick, I just don't believe for one second that that is what killed her. There was blood all over that bedroom, imo. If a child died of a stomach virus in bed, and you were going to "dispose" of her remains via dismemberment, would you do it right there in her bedroom?! No way...not buying this died of natural causes one bit!
ETA: and if there wasn't blood, or semen, or something else all over her mattress, why would it have to be gotten rid of? Couple this with AB's oh so convenient mention of Zahra's entering puberty, and I think it's pretty darned clear he was covering his tracks with a ready made excuse when blood was found. Natural death my eye!