The ancient disease is re-emerging after decades of absence, causing concern among public health experts.

re-emerging 是动词 re-emerge 的现在分词形式。re-emerge 由前缀 re-（表示"再次"）和动词 emerge（出现、浮现）组成，字面意思是"再次出现"。 re-emerging 在这里表示"重新出现"的动作正在进行中。