In the event of a biological attack or the introduction of a highly contagious disease affecting the public, the U.S. health system may take measures to prevent those people infected with or exposed to a disease or a disease-causing biological agent from infecting others. The terms used to describe these measures, quarantine and isolation , generally apply to distinct groups of persons but are often used interchangeably. Quarantine typically refers to the "(s)eparation of individuals who have been exposed to an infection but are not yet ill from others who have not been exposed to the transmissible infection." In contrast, isolation refers to the "(s)eparation of infected individuals from those who are not infected." Varying degrees of quarantine exist, and the authority to order quarantine or isolation is generally very broad. Primary quarantine authority typically resides with state health departments and health officials; however, the federal government has jurisdiction over interstate and foreign quarantine. In addition, the federal government may assist with or take over the management of an intrastate incident if requested by a state or if the federal government determines local efforts are inadequate. The variety of potential responses—from federal, state, and local authorities—implicates a number of legal issues. Namely, who is responsible for preventing an outbreak of the disease; and precisely what may be done? This report will examine federal and state authority to impose quarantine and isolation measures in order to prevent the spread of infectious disease. Federal quarantine and isolation authority derives from the Commerce Clause of the U.S. Constitution, which states that Congress shall have the power "[t]o regulate Commerce with foreign Nations, and among the several states." Section 361 of the Public Health Service Act (PHS Act) grants the Secretary of Health and Human Services (Secretary) the authority to make and enforce regulations necessary "to prevent the introduction, transmission, or spread of communicable diseases from foreign countries into the States or possessions, or from one State or possession into any other State or possession." The law provides the Secretary with broad authority to apprehend, detain, or conditionally release a person. However, the Secretary may only exercise this authority in relation to communicable diseases published in an executive order issued by the President. Executive Order 13295 lists the communicable diseases for which this quarantine authority may be exercised. In 2000, the Secretary transferred certain authorities related to persons, including quarantine authority, to the Director of the CDC. Both interstate and foreign quarantine measures are now carried out by CDC's Division of Global Migration and Quarantine. Generally, federal regulations authorizing the apprehension, detention, examination, or conditional release of individuals are applicable only to individuals coming into a state or possession from a foreign country or possession. To facilitate containment efforts, federal regulations require airline pilots and ship captains to report immediately the presence of ill passengers on board their vessels. In addition, Section 361 of the PHS Act authorizes the apprehension and examination of any individual reasonably believed to be infected with a communicable disease in a qualifying stage and (A) to be moving or about to move from a State to another State; or (B) to be a probable source of infection to individuals who, while infected with such disease in a qualifying stage, will be moving from a State to another State. If found to be infected, such individuals may be detained for such time and in such manner as may be reasonably necessary. During times of war, the authority to apprehend and examine individuals extends to any individual "reasonably believed (1) to be infected with such disease [as specified in an Executive order of the President] and (2) to be a probable source of infection to members of the armed forces of the United States or to individuals engaged in the production or transportation of ... supplies for the armed forces." Regulations promulgated pursuant to this authority under the PHS Act may be found in Parts 70 and 71 of Title 42 of the Code of Federal Regulations. Part 70 applies to interstate travel, Part 71 to foreign arrivals. The implementing regulations authorize the quarantine and isolation of individuals in order to prevent the spread of diseases identified in Executive Order 13295. In addition, pilots of both interstate flights and flights to the United States are required to report certain illnesses they encounter during flight in advance of their arrival into the United States to the CDC Quarantine Station closest to their destination airport. There are currently 20 such stations operated by the CDC located at ports of entry into the United States. While there are not CDC officials at every port of entry, various agencies in the Department of Homeland Security (DHS) are authorized to assist the CDC in "the enforcement of quarantine rules and regulations." The Director of the CDC is also authorized to take measures as may be necessary to prevent the spread of a communicable disease from one state or possession to any other state or possession if he determines that measures taken by local health authorities are inadequate to prevent the spread of the disease. To prevent the spread of diseases between states, the regulations prohibit infected persons from traveling from one state to another without a permit from the health officer of the state, possession, or locality of destination, if such a permit is required under the law applicable to the place of destination. Additional requirements apply to persons who are in the "communicable period of cholera, plague, smallpox, typhus or yellow fever, or who having been exposed to any such disease, is in the incubation period thereof." The Secretary is also authorized to bar the entry of persons from foreign countries where the "existence of any communicable disease" poses a "serious danger" of entering the United States such that "a suspension of the right to introduce such persons and property is required in the interest of public health." A rule implementing this statutory authority was proposed in 2005, but was not adopted. The PHS Act and related statutes also authorize measures to aid or enforce a quarantine in the event of a public health emergency. Section 322(a) of the PHS Act authorizes the PHS to care for and treat persons under quarantine. Such persons may also receive care and treatment at the expense of the PHS from public or private medical facilities when authorized by the officer in charge of the PHS station at which the application is made. Section 311 of the PHS Act provides for federal-state cooperative activities to enforce quarantines. The federal government may assist states and localities in enforcing their quarantines and other health regulations and, in turn, may accept state and local assistance in enforcing federal quarantines. Under the authority of 42 U.S.C. § 97, the Secretary may request the aid of CBP, the Coast Guard, and military officers in the execution of quarantines imposed by states on vessels coming into ports. Criminal sanctions are prescribed for violations of federal regulations issued pursuant to Section 361 of the PHS Act. Violation of a federal quarantine or isolation order is a criminal misdemeanor, and individuals may be subject to a fine of up to $100,000, one year in jail, or both; organizational violations may be subject to fines of up to $200,000 per event. Federal district courts may enjoin individuals and organizations from violation of CDC quarantine regulations. In recent years, federal agencies have developed a travel restriction tool to prevent the spread of communicable diseases of public health significance. The public health Do Not Board (DNB) list was developed by DHS and the CDC, and made operational in June 2007. The DNB list enables domestic and international health officials to request that persons with communicable diseases who meet specific criteria and pose a serious threat to the public be restricted from boarding commercial aircraft departing from or arriving in the United States. The list provides a tool for management of emerging public health threats when local public health efforts are not sufficient to keep certain contagious individuals from boarding commercial flights. In order to place a person on the DNB list, state and local health officials contact their local CDC quarantine station. The CDC determines if the person is (1) likely contagious with a communicable disease that presents a serious public health threat, (2) unaware of or likely not to comply with public health recommendations and medical treatment, and (3) likely to try boarding a commercial aircraft. Once a person is placed on the DNB list, airlines are instructed not to issue a boarding pass to the person for any commercial domestic flight or for a commercial international flight arriving in or departing from the United States. Other forms of transportation, such as buses and trains, are not covered by the DNB list. Once a patient is determined to be noncontagious, the CDC and DHS must remove the person from the list, usually within 24 hours. In September 2008, the CDC released a report in which it analyzed the first year's experience with the DNB list. According to the report, the CDC received 42 requests to add persons to the DNB list, all of whom had suspected or confirmed pulmonary tuberculosis. The agency approved 33 of the requests, of which 28 were placed by public health departments in the United States, and 14 were placed on the list while they were outside of the country. Two of the 33 persons placed on the DNB list attempted to evade the air travel restriction, and both were detained by border officials and taken to local hospitals for evaluation and treatment. The CDC found that "[j]udicious use of the public health DNB list can obviate the human and economic costs associated with conducting contact investigations when people with communicable diseases travel on commercial aircraft." While the federal government has authority to authorize quarantine and isolation under certain circumstances, the primary authority for quarantine and isolation exists at the state level as an exercise of the state's police power. CDC acknowledges this deference to state authority as follows: In general, CDC defers to the state and local health authorities in their primary use of their own separate quarantine powers.