父母赴美是否需健康体检证明

父母赴美 (约半年) 是否需要健康体检,有谁知道吗? 多谢!

My parents are about to go to visit the States. I would like to find out if they are required to present any health paper at the Customs. They plan to stay less than 6 months. Does any one know about this?

Thanks a lot for the help!