The job of quarantining people who are already in the country typically falls to the states. Laws vary, but most state health departments can quarantine first and ask questions later. In this particular case, the decision came from the Texas Department of State Health Services, which sent two county officials and a local deputy to serve the order at Troh’s apartment on Sept. 30, the day a blood test confirmed Duncan’s infection. (In Texas, violating such an order is a third-degree felony; in most other states it’s a criminal misdemeanor.) Health officials are also keeping close tabs on several dozen people who came into contact with Duncan before his symptoms began, but have so far found no reason to extend the quarantine order to include anyone else.

The federal government’s chief responsibility, meanwhile, is preventing infectious diseases from reaching the American public in the first place. The first line of defense are the quarantine stations that the U.S. Centers for Disease Control and Prevention operate at 20 of the most heavily trafficked U.S. ports of call and border crossings. When someone arrives in America who authorities suspect may be infected, they can quarantine the individual until health officials sign off on his or her release.

Such screenings aren’t foolproof. Authorities, for example, didn’t flag Duncan because he denied having contact with anyone infected with Ebola. The federal government announced new screening procedures on Wednesday that will require temperature checks for passengers arriving from the three West African nations that have been hit hardest by the Ebola virus. But that is largely just security theater, since it will do little to stop the spread of the virus. Duncan, for one, would have likely breezed through such a checkpoint because his symptoms did not begin until four days after he arrived in Dallas.