As Gov. Christie fended off criticism for quarantining a nurse just back from aiding Ebola patients in West Africa, the Centers for Disease Control and Prevention released more specific guidelines on how states can monitor returning individuals.

The CDC offered specific risks and response suggestions for travelers without symptoms but did not go as far as to recommend quarantines, such as those imposed Friday by Christie and New York Gov. Andrew Cuomo, which have been called overly cautious.

"New guidelines being posted today increase the level of protection of the health and safety of Americans while at the same time protecting those who are doing the heroic work of protecting us from Ebola as they fight it on the shores of Africa," CDC director Thomas R. Frieden said in a Monday evening conference call.

There are more than 10,000 Ebola cases in Africa, the CDC said.

New York and New Jersey require quarantines of travelers who have been in contact with Ebola patients, regardless of whether the travelers present symptoms. The newly released CDC guidelines are less stringent.

Hours after the governors' announcement Friday, a nurse who had been working in Sierra Leone with Ebola patients landed at Newark Liberty International Airport and became the first person to be quarantined under the new mandate. She was examined after getting off the plane and registered a slight fever.

The nurse, Kaci Hickox, criticized the three-day stay in a University Hospital tent in Newark. She tested negative for the disease Monday and was returned to Maine, where she will continue her quarantine at home. Maine requires a 21-day quarantine from the last exposure to Ebola. Illinois, Florida, Maryland, and Virginia also have home quarantines.

Under the CDC's suggested protocol, Hickox would have posed "some risk" and would not have required quarantine.

"I know she didn't want to be there," Christie said of Hickox. "No one ever wants to be in the hospital, I suspect. And, so, I understand that. But the fact is, I have a much greater, bigger responsibility to the people of the public. So I think when she has time to reflect, she'll understand that as well."

Asymptomatic quarantines have been rebuked by former state health officials, unions, health-care professionals and the American Civil Liberties Union. "We want to make sure these decisions are made based on science and medicine rather than fear and politics," said Alexander Shalom, a senior legal analyst with ACLU New Jersey. "I don't think that's what's happened so far."

Shalom said that while states are fully within their rights to quarantine people, the law requires quarantines be used only when absolutely necessary and in the least restrictive ways.

"It's a huge deprivation of liberty to allow the government to tell you, you can't move around, can't interact with your family, see the people who love you," Shalom said. "We recognize that deprivation is sometimes needed, but we want to put a check on that extraordinary government reaction."

Christie defended his decision while campaigning Monday in Florida and on Twitter, noting the state's preference is to quarantine people at home but that New Jersey took the safest course with Hickox.

"You worry about doing what's right for the people you represent, and that's what we've done," he said on Twitter.

Gov. Corbett said Pennsylvania had no plan to quarantine people without symptoms. Health officials in Pennsylvania said they continue to follow CDC protocol.

"At this time, we believe the procedures we have in place are effective. These are based on CDC recommendations, and we're working very diligently to implement them," said Health Department spokeswoman Aimee Tysarczyk.

As of Monday afternoon, local health workers in Pennsylvania were monitoring 105 people through recommendation of the CDC.

In a Monday news release, the CDC laid out specific risk levels and suggested responses.

An individual who comes in direct contact with an infected person's bodily fluid, for example, is at high risk, and should be actively monitored and avoid travel and public activities.

An individual who has contact with a person with Ebola before the person shows symptoms would be at no risk and would not require monitoring.

According to the CDC, active post-arrival monitoring means that travelers without illness or symptoms consistent with Ebola will be contacted daily by state and local health departments for 21 days from the date of their departure from Liberia, Sierra Leone, or Guinea.

Six states (New York, Pennsylvania, Maryland, Virginia, New Jersey, and Georgia), where approximately 70 percent of incoming travelers are headed, started active monitoring Monday as per the revised CDC guidelines, with the remainder of the states starting this week.

No direct flights from the affected West African nations fly to Newark or JFK airports. Travelers come through connecting destinations.

The CDC also suggested active monitoring of all health-care professionals working with Ebola patients in the United States, regardless of whether hospital protocol was followed.

The quarantine order in New York and New Jersey was specific to health-care workers traveling from the three affected nations but made no mention of workers already in the United States.

George DiFerdinando, New Jersey's deputy health commissioner during the post-9/11 anthrax scare, called the quarantine orders confusing in their preliminary stages. It's unclear what the chain of command will be in terms of moving patients from federal to state to local agencies, said DiFerdinando, who ran New York state's tuberculosis control program from the mid-1980s to mid-1990s, when many people had to be isolated or quarantined.

Most concerning, DiFerdinando said, is the message the quarantine sends to health-care workers - an estimated 300 - who have gone to fight the disease overseas and who could return to a 21-day quarantine.

"We're looking at this backward. The problem is not people coming into Newark and JFK; the problem is getting health-care workers in the United States to fly the other way," DiFerdinando said.

Hickox's accusations that she was mistreated while in quarantine in New Jersey could dissuade returning doctors from being up front with medical professionals, he said.

"You have to do everything you can not to get in an adversarial relationship with the patient," said DiFerdinando, an adjunct professor of epidemiology at Rutgers School of Public Health. "What you're fighting is the disease. If you create a situation where if you give one answer, you're going to go into mandatory quarantine in a big old tent against your will, that might create an incentive to not give a straightforward answer."

In the telephone conference with reporters, CDC head Frieden emphasized that the disease does not spread easily.

"I understand that people are afraid - Ebola is unfamiliar, it is a severe disease - but it is not highly contagious and it requires direct contact with someone who is sick from Ebola or with their bodily fluids," he said.

John Jacobi, professor of health law and policy at Seton Hall University Law School, said that in New Jersey, every community has a public-health agency - either a county or township department. Those agencies would oversee the individual monitoring, he said, meaning any quarantine-staffing costs would likely come from local and state tax dollars.