The 2010 Public Health Service Act authorizes the federal government to prevent the entry and spread of communicable diseases from foreign countries into the United States, as well as between states. That gives the feds pretty broad latitude. Officials at the Centers for Disease Control and Prevention have the power to stop travelers — both foreigners and American citizens — arriving on ships, airplanes and over land. If necessary, travelers may be isolated or quarantined at one of 20 designated quarantine stations. According to an executive order last updated in 2014, the infectious diseases that could prompt CDC to isolate or quarantine an individual are: cholera, diphtheria, infectious tuberculosis, plague, smallpox, yellow fever, viral hemorrhagic fevers, severe acute respiratory syndromes, and new types of flu that could cause a pandemic. COVID-19 is covered under that list because it causes a severe acute respiratory syndrome. Washington also has the power to recommend mandatory vaccination and to determine which groups should get first dibs on the vaccines or drugs that are in short supply. But as a constitutional matter, states can make their own rules on such matters.