The United States, similar to all civilized countries established immigration laws to protect citizens as well as legal immigrants.
Our immigration laws required an inspection, passport and approved visa for legal entries. A person admitted would go through a process that established a clean police record, true name, place of birth, intention of entry, destination, health record, and sponsors if needed. Aliens with communicable or contagious diseases were excluded as well as criminals or the mentally insane. Health issues is a very important part of the admission process.