A resurgence of an almost-eradicated, highly contagious disease in the Northern Territory has now spread across three state borders, in what federal Health Minister Mark Butler has described as a "very concerning" development.The Northern Territory has recorded 133 cases of diphtheria to date, prompting a major push to increase vaccination rates amongst those in the hardest-hit communities.One person in the Territory has died, although the exact cause of death remains under investigation.READ MORE: Accused domestic violence killer was 'full-time carer'