It was nearly 100 years ago that an influenza pandemic led to sweeping quarantines in American cities, and it was more than two decades ago that patients in New York were forced into isolation after an outbreak of tuberculosis.

In modern America, public health actions of such gravity are remarkably rare. So the decisions by New York and New Jersey on Friday to quarantine some travelers returning from the Ebola zone in West Africa have taken public officials into unfamiliar legal and medical territory.

From public health advocates and civil liberties lawyers has come sharp criticism, and the first person to be detained under the new protocol, a nurse who was quarantined in New Jersey upon returning from Sierra Leone, lashed out on Sunday at Gov. Chris Christie as her lawyer said he would mount a legal challenge to her confinement.

Mr. Christie on Sunday defended his policy, and Florida and Illinois have ordered similar restrictions. In Connecticut, nine people who may have been exposed to Ebola have been confined to their homes under an order signed by Gov. Dannel P. Malloy on Oct. 7.