Story Highlights • CDC: Airline passengers may have been exposed to drug-resistant TB

• Two flights involved: Air France 385 on May 12; Czech Air 0104 on May 24

• Exams urged for passengers in same row, two rows ahead or behind

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ATLANTA, Georgia (CNN) -- Federal health authorities said Tuesday that they are looking for people who may have been exposed to a rare and potentially fatal form of tuberculosis from an infected passenger during two trans-Atlantic flights this month.

The man, infected with the extensively drug-resistant form of TB known as XDR TB, departed Atlanta, Georgia, on May 12 aboard Air France Flight 385 and arrived in Paris, France, the next day, said Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention.

The man, who has not been identified publicly, returned last Thursday to North America aboard Czech Air Flight 0104 from Prague, Czech Republic, to Montreal, Canada, then drove into the United States.

"During these two long flights, the patient may have been a source of infection to the passengers," Gerberding told reporters.

Those most at risk would have been seated within two rows of the man, she said, adding that she was not sure what seats he occupied. Still, she said, CDC is recommending that all passengers be notified.

"I don't think we would compel people to be tested, but we would strongly recommend" that anyone seated near the man undergo a baseline test now and a follow-up test several weeks from now, she said.

Final diagnosis for XDR TB can take six to 16 weeks.

XDR TB was recently defined as a subtype of multiple-drug resistant tuberculosis. It often proves fatal.

The man's tuberculosis had been diagnosed before his departure, but he disregarded his doctors' recommendation that he not travel, she said. "The patient had compelling reasons for traveling and made the decision to go ahead and meet those personal responsibilities," she said, adding that federal authorities did not know until he had left the country that he had the rare form of the disease. The man himself may not have known either, she said.

About his reasons for traveling, she said, "They were compelling, from his perspective, and we understand and certainly respect that."

Once he returned, the man was ordered into isolation, "and is required to stay in isolation until the responsible public health officials deem that he is no longer infectious to others," she said.

Treated in Atlanta

He is being treated at Grady Memorial Hospital in Atlanta, Georgia, a spokeswoman for the hospital said.

He is a resident of Fulton County, Georgia, Gerberding said.

Because the man's sputum does not test positive under the microscope for the bacillus, the chance that he would infect others is considered low, Gerberding said. "But we know it isn't zero," she said, adding that 17 percent of TB cases are transmitted by such carriers.

A spokesman for the Public Health Agency of Canada said the CDC contacted the man while he was on vacation with his family in Europe and told him not to return on a commercial flight.

Though the man ignored that caution, spokesman Dr. Howard Njoo noted that planes are equipped with air filters that are designed to trap the long, rod-shaped tuberculosis bacilli.

The man returned via Canada and entered the United States by driving through the border crossing at Champlain, New York, Njoo said.

Customs and Border Protection spokesman Kevin Corsaro said the man did not appear sick to border agents.

CBP said it has not changed its screening or security precautions as a result of the case.

Once he returned to the United States, the man was contacted by health officials, who required that he go to an isolation hospital in New York City for evaluation, said Dr. Martin Cetron, the CDC's chief of quarantine.

"He drove himself there, voluntarily."

The man was admitted and served a provisional quarantine order that lasted 72 hours while he was being assessed, Cetron said.

Asked whether he preferred to stay in New York or return to his family in Atlanta for treatment, the man chose the latter option, said Cetron. CDC officials then arranged for the patient to be flown Monday aboard a CDC plane, an unusual use of agency resources, Gerberding acknowledged.

She said the move was "one that we felt was fair and appropriate, given that he is a citizen of Georgia, his family members are here and his disease does require prolonged treatment."

Upon his arrival in Atlanta, he was issued a federal isolation order to cover the time while the case was handed over to the jurisdiction of state and local officials in Fulton County, Cetron said.

"That process of the handoff is ongoing right now."

XDR TB rare in the U.S.

The patient, who has few symptoms, has X-ray evidence of pulmonary TB and tests positive for XDR TB, the agency said.

Between 1993 and 2006, XDR TB was diagnosed in 49 people in the United States, said Dr. Ken Castro, director of the division of TB Elimination at CDC.

But the disease is more common elsewhere, he said. "When they looked, they found it in every single continent of the world," he said.

One in three people in the world is infected with dormant TB bacteria, according to the World Health Organization. Age, immune suppression and other medical conditions can activate the bacteria, which can usually be treated with a course of four standard, or first-line, anti-TB drugs.

WHO estimates that there were almost half a million cases of multiple-drug-resistant tuberculosis worldwide in 2004.

People with TB of the lungs, the site most commonly affected, can spread the disease by coughing, sneezing or even talking.

"A person needs only to breathe in a small number of these germs to become infected (although only a small proportion of people will become infected with TB disease)," the WHO said on its Web site.

"The risk of becoming infected increases the longer the time that a previously uninfected person spends in the same room as the infectious case," it added.

People with XDR TB are resistant to first- and second-line drugs; their treatment options are limited.

Crowded, poorly ventilated, closed environments are most conducive to the spread of infection, it said. Cure is possible for up to 30 percent of cases, it added.

People with HIV infection and other diseases that suppress the immune system are most at risk of catching TB and becoming sick.

No one at the disease agency recalls the agency issuing a quarantine order since 1963, when a possible case of exposure to smallpox emerged, she said.

"From our perspective, no laws were broken here," Gerberding said. "Our system works very well."