Why the government has legal authority to quarantine

Under section 361 of the Public Health Service Act (42 U.S. Code § 264), the U.S. Secretary of Health and Human Services is authorized to take measures to prevent the spread of communicable diseases into the United States and between states, according to the Centers for Disease Control and Prevention (CDC), which holds the authority for carrying out these functions…

Isolation and quarantine can be imposed by states under their police power functions, which give them the right to protect the health and safety of people within their borders. The four people currently quarantined in Texas are Duncan’s close family members. They received orders Wednesday from Texas and Dallas County officials “not to leave the apartment or to receive visitors without approval” until October 19, TIME’s Alex Altman reports.

But if states’ powers are not enough to stem the spread of a disease, the federal government can also institute isolation and quarantine. The last time this was used on a large scale was during the “Spanish Flu” pandemic in 1918-1919, according to the CDC.