退化 的英文怎麼說?

[1] to regress[2] to return to a previous and worse state[3] to revert to an earlier and less advanced state

regress = 退化，也就是倒退到之前的更糟的狀況，常用在健康與疾病，例如:

regress to the mental age of a kid = 智商退化到孩子的程度

regress 也有「惡化」的意思，例如:

He's diagnosed with cancer a year ago, and it regressed rapidly (病情迅速惡化).