As word spread Thursday night that a New York doctor who recently treated Ebola patients in West Africa was now infected with the disease, many began to wonder why he hadn’t been quarantined immediately upon his return to a densely populated city of 8 million people.

Those concerns only grew as it was revealed that the doctor, 32-year-old Craig Spencer, rode the subway, visited a public park, ate at a restaurant and went bowling in New York over the two days before he began showing symptoms.

Many others had the same thought.

I’m stunned there’s no quarantine, if not for all, then at least high risk people (doctors), coming back from Ebola stricken countries.

— E. K. Holbrook (@Erknho) October 24, 2014

I’m all for doctors aiding those suffering from #Ebola in Africa. But upon returning to the U.S., those doctors should self-quarantine.

— JayhawkMama (@JHawkintheSouth) October 24, 2014

But Spencer, officials say, did everything right.

He checked his temperature frequently, monitored himself for symptoms, stayed close to a hospital with isolation facilities and, when he developed a fever nine days after his return, immediately contacted Doctors Without Borders, the organization that sent him to Guinea, one of the three nations in West Africa experiencing a severe Ebola epidemic.

He did exactly the right thing- contacted MSF, and they contacted us immediately @HHCnyc @CDCgov @BilldeBlasio pic.twitter.com/A58EQa0tkx

— Dr. Mary Bassett (@DrMaryTBassett) October 24, 2014

But why wasn’t the doctor, who had direct contact with Ebola-infected patients in West Africa, in quarantine as soon as he got back to New York? It’s simple: He was following guidelines set by Doctors Without Borders, also known by its French initials MSF.

“Self quarantine is neither warranted nor recommended when a person is not displaying Ebola-like symptoms,” an MSF spokesman said when asked about the policy. “Normal life can proceed” as long as a staff member who returns from an Ebola-affected area shows no symptoms, he said, adding that MSF’s guidelines are “consistent with those provided by the CDC.”

Until Thursday in New York, those guidelines worked. Of more than 700 MSF staffers deployed to the region, none had ever returned home and later developed Ebola.

Now, though, that may change.

New federal guidelines start Monday

Federal health officials announced on Wednesday that all travelers from West African countries affected by the Ebola outbreak will be monitored for 21 days upon entering the United States, in a new program scheduled to start Monday.

The new measures, announced by Tom Frieden, the director of the Centers for Disease Control and Prevention (CDC), represent a significant escalation in the government’s attempts to reduce the risk of additional Ebola cases in the U.S.

According to Frieden, the level of “active monitoring” will depend on whether a traveler had close contact with an Ebola patient. Someone who had little to no interaction with Ebola patients would have to self-report body temperatures to local health authorities, while someone who had significant exposure to a sick person’s bodily fluids would be fully quarantined.

Active monitoring of all people returning from hot zones starting; state/local public health essential to find and stop #Ebola.

— Dr. Tom Frieden (@DrFriedenCDC) October 24, 2014

The monitoring window is three weeks long because that is the longest known period of incubation for the Ebola virus.

“There are a variety of situations and it would be individualized,” Frieden said during a conference call with reporters. “We will require a quarantine for the highest risk group.”

That, it would seem, would include Spencer.

The new monitoring will be carried out by six states, including New York. New York City’s health department will directly monitor anyone who returns from Liberia, Guinea and Sierra Leone, said Dr. Mary Travis Bassett, the city’s health commissioner.

“We and other localities are going to be monitoring any individual who has returned from those nations.” – Mayor @BilldeBlasio.

— NYC Mayor’s Office (@NYCMayorsOffice) October 24, 2014

“We used to recommend to people that they voluntarily monitor” their symptoms, Bassett said Friday, but now the city is escalating its procedures. “Now we’re going to call them,” Bassett said. “That’s what we mean by active monitoring.”

Stricter quarantine being discussed

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Friday that a stricter quarantine for people coming from West Africa is “something that is under active discussion.”

“You’ll be hearing shortly what the guidelines will be,” Fauci said, adding that the states have an awful lot to say on the matter, as federal quarantine laws generally defer to local authorities.

Sen. David Vitter (R-La.) has been calling for quarantine for travelers from Africa since last week.

“The CDC director’s response to the spread of the Ebola virus has been nothing short of pathetic,” Vitter said in a statement on Oct. 16.

“President [Barack] Obama needs to fire him, put someone strong and competent in charge, and impose travel restrictions into the U.S. — all immediately. That sort of quarantine action was an essential part of the eradication of the late 1970s Ebola outbreak in Africa.”

Some doctors have already self-quarantined

At least one doctor who returned from the region in August decided to put himself under quarantine in a display of over-caution.

Dr. Alan Jamison, 69, volunteered in Liberia and returned to the U.S. in late July and stayed isolated for the full 21 days.

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“I’m feeling normal and doing the typical things a person would do in their home,” he told CNN at the time. “I have my family who can bring me food if I need anything, and they would not enter the house. They can leave items outside the home.”

“I was not concerned that I was contagious when I left Africa, and not concerned at this time because I have no symptoms of the disease,” Jamison said.

Workers can “go about their lives”

For New York Mayor Bill de Blasio, it doesn’t make sense to isolate every health care worker who has treated an Ebola patient. After all, he said Friday, “hundreds” of workers have been involved in the care of Ebola patients at specially designated U.S. hospitals, and none have gotten sick.

de Blasio: We learned from Dallas. The level of preparation and training is an entirely different dynamic.

— MashableLIVE (@MashableLive) October 24, 2014

Likewise, the doctors and nurses now caring for Spencer at Bellevue Hospital “can go about their lives” outside of work, he said.

Officials say they don’t know how Spencer was infected during his work in West Africa; he believed his protective equipment had not been breached.

Critics have said that Spencer should have been more careful after coming in contact with Ebola patients in a less-controlled environment. But de Blasio emphasized the importance of the work of doctors like Spencer in Africa, saying that the only way to truly protect Americans from Ebola was to stop the outbreak at its source.

“These individuals who are going there to serve are the people who will end this crisis, let’s be clear,” he said.

Andrew Freedman and Jonathan Ellis contributed to this report.

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