Health,
1.) HIV was brought to the U.S.A by a European
2.) Nobody knew what HIV was and being that it kills by attacking the immune system it’s symptoms are not readily apparent as a new virus and by the time it was already recognized as an merging virus it was already imbedded in the population
3.) it is not possible to quarantine a country and anyone that travels to one. It’s just not practical unless you would stop ALL immigration and ALL travel from any country on earth. All that restricting travel would do is make it impossible for foreign doctors to fight the virus and let it evolve and mutate further into more dangerous/ infectious strains.