WASHINGTON (Reuters) - The United States has placed in isolation a man who may have exposed fellow passengers and crew on two May trans-Atlantic flights to a tuberculosis strain that is extremely hard to treat, officials said on Tuesday.

It was first time the U.S. government has issued such an isolation order since 1963, when it took action against a smallpox patient, according to the U.S. Centers for Disease Control and Prevention.

The man, a U.S. citizen from the state of Georgia, is suffering from extensive drug-resistant TB, known as XDR TB, which resists virtually all antibiotics, the CDC said. Tuberculosis is a bacterial infection that usually attacks the lungs.

CDC Director Dr. Julie Gerberding said authorities in the United States and other countries were trying to notify passengers who traveled on Air France flight 385, arriving in Paris from Atlanta on May 13, and on Czech Air flight 0104, arriving in Montreal from Prague on May 24.

Gerberding said health authorities were aware of the man’s condition before he left the United States and warned him against traveling.

“Under the circumstances, I think we were surprised that the patient had left the country,” she said.

She said the man had “compelling personal reasons” to travel despite being ill. “I want to emphasize that from our perspective, no laws were broken here,” Gerberding said.

The man returned to the United States by car. He voluntarily entered a medical isolation facility in New York City on Friday before being flown on a CDC plane back to Atlanta on Monday. Authorities called him “relatively asymptomatic.”

SOME PASSENGERS AT RISK

Officials did not give the patient’s name or age or say where he became infected. Gerberding said he must remain isolated until public health officials deem him to be no longer infectious.

Tuberculosis kills about 1.6 million people annually, with the highest number in Africa. It is spread through the air when infectious people cough, sneeze, talk or spit.

Some TB bacilli acquire the ability to beat most medications, requiring extra medical efforts that often are impossible in the developing nations where tuberculosis exacts its highest death toll.

“This is an unusual TB organism, one that’s very, very difficult to treat. And we want to make sure that we have done everything we possibly can to identify people who could be at risk,” Gerberding told a news conference.

The passengers most likely to be at risk were those seated within two rows of the patient, Gerberding said. She did not say how many passengers and crew were on the flights or how many countries had citizens aboard.

People who think they were on one of the affected flights should be tested for TB, the CDC said.

CDC said it was working with U.S. state and local health departments, ministries of health in other countries, the airline industry and the World Health Organization.

Dr. Kenneth Castro of the CDC’s Division of Tuberculosis Elimination said that from 1993 to 2006 CDC knew of 49 people in the United States with XDR TB.

XDR TB requires 18 months to two years of treatment with a mixture of four to six drugs. The treatment can often require surgery, as well, and can cost $500,000 per patient.

XDR TB is resistant to first-line antibiotics, and to an entire class of antibiotics called fluoroquinolones, as well as to at least one of three injectable drugs.

Air France-KLM said it is “cooperating fully with all authorities involved.”