That is the problem when you have fully open borders. Notice Singapore have installed temperature sensing equipment at entry points, so they can check each arrival's body temp. Airport operators, airlines and Governments together are going to have to come up with some solutions to prevent this happening again. Take a look how a Sky journalist re-entered China, hoping to go to Beijing and ended in some location away from there in a 14 day lockdown and checked many times by people in full Hazmat gear! Now if China is doing that to prevent people bringing the virus back in to their country, then they must know much more about it than what they have released to the rest of the world. As for the World Health Organisation, less said just another agency without "many teeth".