US officials have begun a worldwide search for people who may have come into contact with a man infected with a drug-resistant form of tuberculosis.
Tuberculosis is a bacterial infection spread through the air
They say crew and passengers on the same flights as the man, from Atlanta to Paris and from Prague to Montreal this month, should be checked.
The man was honeymooning in Italy when he was told he had the rare form of TB - but still decided to return home.
He is now under federal quarantine, the first issued in over 44 years.
The Centers for Disease Control and Prevention (CDC) has identified the illness as "extensively drug-resistant TB".
CDC director Julie Gerberding said it was looking through passenger lists to try to find people who may have come into contact with the man, whose name has not been made public.
"Is the patient himself highly infectious? Fortunately, in this case, he's probably not...," Dr Gerberding said, adding that the bacteria was "very deadly". "We just have to err on the side of caution."
The man told the Atlanta Journal-Constitution that he knew he had a form of tuberculosis but not that it was so serious. He told the paper doctors had not told him to avoid flying, the Associated Press said.
When he was on honeymoon he was told by health officials that he should surrender himself to Italian authorities.
"I thought to myself: You're nuts. I wasn't going to do that. They told me I had been put on the no-fly list and my passport was flagged," the man said.
The last federal quarantine order was issued in 1963, to isolate a patient with smallpox, according to the CDC.
Tuberculosis is a bacterial infection that usually attacks the lungs. It is spread through the air and can lead to symptoms such as chest pain and coughing up blood.
There were an estimated 1.6 million deaths from TB in 2005, according to the World Health Organization (WHO).
The infected man travelled from Atlanta to Paris on 12 May on Air France flight 385. He returned to the North America on CSA flight 104 from Prague to Montreal on 24 May and continued his journey by car.
CDC officials said the man was potentially infectious during this period and are recommending that crew members and passengers on board the same flights seek medical attention.
"We want to make sure that we have done everything we possibly can to identify people who could be at risk," said Dr Gerberding.
TB is rare in the US. Last year there were 13,767 recorded cases or 4.6 cases per 100,000 Americans.
About 1.2% of cases in the US are "multidrug-resistant", and can withstand antibiotics commonly used to treat the illness, according to CDC statistics.
The "extensively drug-resistant" TB is more dangerous. Medical treatment can cost $500,000 (£250,000) or more, CDC officials say.